87 S4 fuel delivery problem - too rich!
03-28-2002, 01:12 PM
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87 S4 fuel delivery problem - too rich!
Hello
My '87 starts okay but idles so rough that the car does not move at all when any gear is selected (Auto)! It wants to die when the gas pedal is pressed. It stambles, shakes, and is ready to stall. I thought at first what it not getting enough fuel, but the opposite is true.
I have replaced the following:
a) fuel pressure regulator (new)
b) both fuel dumpers (new)
c) fuel relay (used), and fuel fuse (new)
d) fuel pump (used) - the old one works good also
e) new spark plugs (old ones fouled - black tips)
With a fuel pressure meter hooked up it shows pressure at 60 !! Obviously too high...
What else can I check to determine where the problem is??
How the fuel pressure is controled besides the regulator?? Which of the two brains controls that??
Is there a way to check the ECU brain connectors ?? and which connectors ??
Thanks for your time...Vasilis
P.S. If you don't know the answer(s) to this problem, please don't respond...Smart remark like "buy the workshop manuals", "check with your Porsche dealer" and the like, are not appreciated... Veterean 928 users with similar problems in the past need to respond. All others just READ and LEARN!
<img src="graemlins/icon501.gif" border="0" alt="[icon501]" />
My '87 starts okay but idles so rough that the car does not move at all when any gear is selected (Auto)! It wants to die when the gas pedal is pressed. It stambles, shakes, and is ready to stall. I thought at first what it not getting enough fuel, but the opposite is true.
I have replaced the following:
a) fuel pressure regulator (new)
b) both fuel dumpers (new)
c) fuel relay (used), and fuel fuse (new)
d) fuel pump (used) - the old one works good also
e) new spark plugs (old ones fouled - black tips)
With a fuel pressure meter hooked up it shows pressure at 60 !! Obviously too high...
What else can I check to determine where the problem is??
How the fuel pressure is controled besides the regulator?? Which of the two brains controls that??
Is there a way to check the ECU brain connectors ?? and which connectors ??
Thanks for your time...Vasilis
P.S. If you don't know the answer(s) to this problem, please don't respond...Smart remark like "buy the workshop manuals", "check with your Porsche dealer" and the like, are not appreciated... Veterean 928 users with similar problems in the past need to respond. All others just READ and LEARN!
<img src="graemlins/icon501.gif" border="0" alt="[icon501]" />
03-28-2002, 01:49 PM
#2
Your fuel pressure really isn't too high, and is not the source of your car running too rich. Even if the fuel pressure was a lot higher, the fuel injection system should adjust the mixture to where it's supposed to be by varying the injector pulse width. Your problem is most likely a bad oxygen sensor, bad MAF sensor, or a bad LH computer.
03-28-2002, 01:56 PM
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Hi Vasilis--
Fuel pressure varies with manifold pressure, so that it goes higher as manifold pressure rises. High fuel press might be related to a leak in the vacuum line from the manifold to the regulator, or a failure of the fuel pressure regulator itself.
First step would be to check to make sure you have vacuum at the FP hose with the engine running. A gauge would be handy, but you should be able to just use your finger on the end of the hose.
Since you have the fuel pressure gauge, you can test this by watching the pressure as you change load. A hard blip of the throttle at idle causes manifold pressure to rise, and should cause fuel pressure to rise at the same time. If it doesn't change that way, go after the regulator itself.
The brain doesn't control pressure, except to provide the run signal for the relay that starts the pump.
Hope this helps!
Fuel pressure varies with manifold pressure, so that it goes higher as manifold pressure rises. High fuel press might be related to a leak in the vacuum line from the manifold to the regulator, or a failure of the fuel pressure regulator itself.
First step would be to check to make sure you have vacuum at the FP hose with the engine running. A gauge would be handy, but you should be able to just use your finger on the end of the hose.
Since you have the fuel pressure gauge, you can test this by watching the pressure as you change load. A hard blip of the throttle at idle causes manifold pressure to rise, and should cause fuel pressure to rise at the same time. If it doesn't change that way, go after the regulator itself.
The brain doesn't control pressure, except to provide the run signal for the relay that starts the pump.
Hope this helps!
03-28-2002, 02:58 PM
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Thanks for the reply..
Mike -
If the fuel pressure is not too high, then what's the normal fuel pressure reading at idle..Reading other people's posts I thought 60psi was high..Anyway, if you or anyone else knows what the correct value is for the S4 models please post.
Personally I really doubt that a bad oxygen sensor will make the car idle so rough...We're talking about the whole car is shaking and does not move when you step on the gas...I had my oxygen sensor disconneted on my '83 (by accident) and I hardly noticed it until a week later! So I think I can rule out the oxygen sensor.
Dr. Bob -
If I read your post correctly, you're saying the LH brain DOES NOT control fuel pressure, right?? Which is contradictory to what Mike is suggesting on his post....Okay, so what gives???
I would hate to look for another LH brain (they are not cheap) just to find out it's something simple...And how do I determine the LH brain is working correctly or not???
Anyone has a procedure to test the LH brain??
Thanks again...
Mike -
If the fuel pressure is not too high, then what's the normal fuel pressure reading at idle..Reading other people's posts I thought 60psi was high..Anyway, if you or anyone else knows what the correct value is for the S4 models please post.
Personally I really doubt that a bad oxygen sensor will make the car idle so rough...We're talking about the whole car is shaking and does not move when you step on the gas...I had my oxygen sensor disconneted on my '83 (by accident) and I hardly noticed it until a week later! So I think I can rule out the oxygen sensor.
Dr. Bob -
If I read your post correctly, you're saying the LH brain DOES NOT control fuel pressure, right?? Which is contradictory to what Mike is suggesting on his post....Okay, so what gives???
I would hate to look for another LH brain (they are not cheap) just to find out it's something simple...And how do I determine the LH brain is working correctly or not???
Anyone has a procedure to test the LH brain??
Thanks again...
03-28-2002, 04:18 PM
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Vasilis wrote:
[quote] Dr. Bob -
If I read your post correctly, you're saying the LH brain DOES NOT control fuel pressure, right?? Which is contradictory to what Mike is suggesting on his post....Okay, so what gives???
I would hate to look for another LH brain (they are not cheap) just to find out it's something simple...And how do I determine the LH brain is working correctly or not???
<hr></blockquote>
The LH brain does NOT control fuel pressure. What it does do is control the amount of fuel delivered to the engine, based on the pulse length of the signals to the injector nozzles relative to load and RPM. That's what Mike is saying.
Mike is correct that the 60 PSIG you are seeing is probably not excessive enough to cause severe symptoms. It's about what you might expect to see if you are checking the fuel pressure with the engine not running, fuel pump relay bypassed. Are you checking with the engine off, or is it running at idle?
In your original post, you asked that we refrain from suggesting that you should read the manuals, but we are getting very close to that point. The possible problems get narrowed down to sensors that attach to the brain, and the brain itself.
The easiest/fastest diagnosis is to find a nearby owner who will allow you to temporarily swap in your car's brain and see if the symptoms go with it. If the brain works OK in the other car, you will start the hunt for the sensor that's beating you up.
The service manual gives you some good starting points, easy stuff like the temp sensor II that lives on the top of the intake. Bad sensor or bad connection/wiring might be telling the engine that it's stone cold, a condition that normally requires a rich mixture. The airflow sensor (MAS) fails into a limp-home mode, and that tends to be rich rather than lean. There's a full-throttle position sensor switch that might include some artificial enrichment if the brain perceives that the throttle is full open under low rpm's and low load. Again, the service manual walks you through the expected signals from these sensors, except for the MAS.
There is a certain false logic that suggests that the $300-400 cost of manuals might not be a good investment. Here's a situation where quite a bit of time might be saved having the procedures in front of you. I'm not at home so I can't quote you the steps, and can't fax you the pinouts on the brain to help you with the sensor checks. Result is that you can potentially continue the scattergun approach of replacing components arbitrarily until the problem goes away. Plus there's the time invested. I can always make more money, but time is a very non-renewable resource.
The LH brain is a known weak spot on the S4 cars. I have a loaner brain on my workbench shelf that is in line for in inspect/repair if I can find the time soon. Otherwise it will probably get shipped back to its owner for use as a core or a paperweight. No benefit to you either way...
There was a post to Rennlist in the last few weeks about a service in the midwest that repairs the brains, cost was ~$600 if I remember. Do a search on LH and brain to find it.
Still, the quick-and-dirty process of elimination has you trying either your brain in a loaner car, or a loaner brain in your car.
There's also the possibility of taking the car to a 928 mechanic and having the diagnosis done for you. It may not be cheap, may not be fast, might be accurate and a time and money saver in the long run. Lots of us poo-poo the dealer about repairs, but there comes a time when having an expert with the right tools is a net bargain. Just a thought...
Let us know what you discover, please.
[quote] Dr. Bob -
If I read your post correctly, you're saying the LH brain DOES NOT control fuel pressure, right?? Which is contradictory to what Mike is suggesting on his post....Okay, so what gives???
I would hate to look for another LH brain (they are not cheap) just to find out it's something simple...And how do I determine the LH brain is working correctly or not???
<hr></blockquote>
The LH brain does NOT control fuel pressure. What it does do is control the amount of fuel delivered to the engine, based on the pulse length of the signals to the injector nozzles relative to load and RPM. That's what Mike is saying.
Mike is correct that the 60 PSIG you are seeing is probably not excessive enough to cause severe symptoms. It's about what you might expect to see if you are checking the fuel pressure with the engine not running, fuel pump relay bypassed. Are you checking with the engine off, or is it running at idle?
In your original post, you asked that we refrain from suggesting that you should read the manuals, but we are getting very close to that point. The possible problems get narrowed down to sensors that attach to the brain, and the brain itself.
The easiest/fastest diagnosis is to find a nearby owner who will allow you to temporarily swap in your car's brain and see if the symptoms go with it. If the brain works OK in the other car, you will start the hunt for the sensor that's beating you up.
The service manual gives you some good starting points, easy stuff like the temp sensor II that lives on the top of the intake. Bad sensor or bad connection/wiring might be telling the engine that it's stone cold, a condition that normally requires a rich mixture. The airflow sensor (MAS) fails into a limp-home mode, and that tends to be rich rather than lean. There's a full-throttle position sensor switch that might include some artificial enrichment if the brain perceives that the throttle is full open under low rpm's and low load. Again, the service manual walks you through the expected signals from these sensors, except for the MAS.
There is a certain false logic that suggests that the $300-400 cost of manuals might not be a good investment. Here's a situation where quite a bit of time might be saved having the procedures in front of you. I'm not at home so I can't quote you the steps, and can't fax you the pinouts on the brain to help you with the sensor checks. Result is that you can potentially continue the scattergun approach of replacing components arbitrarily until the problem goes away. Plus there's the time invested. I can always make more money, but time is a very non-renewable resource.
The LH brain is a known weak spot on the S4 cars. I have a loaner brain on my workbench shelf that is in line for in inspect/repair if I can find the time soon. Otherwise it will probably get shipped back to its owner for use as a core or a paperweight. No benefit to you either way...
There was a post to Rennlist in the last few weeks about a service in the midwest that repairs the brains, cost was ~$600 if I remember. Do a search on LH and brain to find it.
Still, the quick-and-dirty process of elimination has you trying either your brain in a loaner car, or a loaner brain in your car.
There's also the possibility of taking the car to a 928 mechanic and having the diagnosis done for you. It may not be cheap, may not be fast, might be accurate and a time and money saver in the long run. Lots of us poo-poo the dealer about repairs, but there comes a time when having an expert with the right tools is a net bargain. Just a thought...
Let us know what you discover, please.
03-28-2002, 04:53 PM
#6
With the fuel pump relay jumped so that the pump runs, and no vacuum going to the fuel pressure regulator, the specifications for the fuel pressure say it should be 53-59 psi.
The LH brain does not control fuel pressure. It controls the amount of fuel delivered to the engine by varying how long the fuel injectors are opened. The only thing that controls fuel pressure is the fuel pressure regulator, and it will vary the fuel pressure depending on how much vacuum there is going to it.
There are various sensors that dr bob mentioned that could be bad, but from what I've seen most of them going bad won't make the car run as badly as what your description sounds like. When it's running as bad as you said, and is excessively rich, the problem is usually a bad MAF sensor or bad LH computer. The only way to really check the LH computer is to switch with another one. If you do this, put your LH computer into another car, instead of putting the other car's computer into your car. The reason for this is that if your car has some wiring or other problem, the good LH computer can potentially be damaged by putting it into your car. If your LH is bad and you put it into another car, nothing can be damaged, but the other car will run poorly with your computer in it.
The LH brain does not control fuel pressure. It controls the amount of fuel delivered to the engine by varying how long the fuel injectors are opened. The only thing that controls fuel pressure is the fuel pressure regulator, and it will vary the fuel pressure depending on how much vacuum there is going to it.
There are various sensors that dr bob mentioned that could be bad, but from what I've seen most of them going bad won't make the car run as badly as what your description sounds like. When it's running as bad as you said, and is excessively rich, the problem is usually a bad MAF sensor or bad LH computer. The only way to really check the LH computer is to switch with another one. If you do this, put your LH computer into another car, instead of putting the other car's computer into your car. The reason for this is that if your car has some wiring or other problem, the good LH computer can potentially be damaged by putting it into your car. If your LH is bad and you put it into another car, nothing can be damaged, but the other car will run poorly with your computer in it.
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03-28-2002, 05:09 PM
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Great job Mike and Bob.
I will follow your advise and look for another S4 in my area (I don't know of anyone off hand in Upstate NY - Dutchess County area) and try the swap of the LH unit you suggested.
I will report back the results....
Bob, if you can fax me or email me the pinouts of the brain, that would be great...I don't have any Workshop manuals. My fax is: 845-432-8264.
Thank you both for the lenghty explanations...I think I have a better understanding of the different components.
Regards
Vasilis Karras
I will follow your advise and look for another S4 in my area (I don't know of anyone off hand in Upstate NY - Dutchess County area) and try the swap of the LH unit you suggested.
I will report back the results....
Bob, if you can fax me or email me the pinouts of the brain, that would be great...I don't have any Workshop manuals. My fax is: 845-432-8264.
Thank you both for the lenghty explanations...I think I have a better understanding of the different components.
Regards
Vasilis Karras
03-28-2002, 06:07 PM
#9
Three Wheelin'
Sounds like when the MAF sensor went bad on my '87. Terrible idle, dies when gas applied, and blows out all sorts of black smoke to make the eyes water. Dave Roberts at 928 Specialists sent me a loaner and it fired right up. Bought the new one for $500 and sent the loaner back. When the brain gets fried, I don't think the car will even start.
03-28-2002, 06:30 PM
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dr bob wrote:
[quote] I'm not at home so I can't quote you the steps, and can't fax you the pinouts on the brain to help you with the sensor checks. <hr></blockquote>
Vasilis replied:
[quote] Bob, if you can fax me or email me the pinouts of the brain, that would be great...I don't have any Workshop manuals. My fax is: 845-432-8264.
<hr></blockquote>
dr bob replies:
Sorry, won't be home until Saturday or Sunday.
by remote control from Phoenix, Az--
[quote] I'm not at home so I can't quote you the steps, and can't fax you the pinouts on the brain to help you with the sensor checks. <hr></blockquote>
Vasilis replied:
[quote] Bob, if you can fax me or email me the pinouts of the brain, that would be great...I don't have any Workshop manuals. My fax is: 845-432-8264.
<hr></blockquote>
dr bob replies:
Sorry, won't be home until Saturday or Sunday.by remote control from Phoenix, Az--
03-28-2002, 06:40 PM
#11
Different cars have responded differently when the LH computer was bad. Some have just died and wouldn't start anymore. Others have had idling and running problems to various degress, and have been driven for a year or more with a bad or failing LH computer.
03-28-2002, 10:08 PM
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I’m with Doug, MAF sensor. When it fails the cars engine management system call for a rich mixture. Limp home mode.
With a voltmeter on terminals 4 and 2 of the disconnected plug of the MAF sensor, jump terminals 30 and 87 of relayXXV, = 12.V.
Ohms check at the sensor. Terminals 3 and 6= 0 to 1000 OHMS
Terminal 3 and 5= 3.6 to 4.1 OHMS
Hot wire check; jump terminals 30 and 87 of relay XXV on the central board
Voltmeter to terminals 3 and 5 of the sensor = 1 to 5 volts
With a blow dryer, blow cold air through the sensor = a rise in voltage
If you can limp your car over to Orange County, you can use my sensor as a test.
Steve C
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<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
With a voltmeter on terminals 4 and 2 of the disconnected plug of the MAF sensor, jump terminals 30 and 87 of relayXXV, = 12.V.
Ohms check at the sensor. Terminals 3 and 6= 0 to 1000 OHMS
Terminal 3 and 5= 3.6 to 4.1 OHMS
Hot wire check; jump terminals 30 and 87 of relay XXV on the central board
Voltmeter to terminals 3 and 5 of the sensor = 1 to 5 volts
With a blow dryer, blow cold air through the sensor = a rise in voltage
If you can limp your car over to Orange County, you can use my sensor as a test.
Steve C
The Great White
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
03-28-2002, 11:16 PM
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Ahhhhh, the joys of the Bosch Injection System.
Vasilis-
If you get the chance to check your LH unit - be certain to use your unit in the donor car - and not the other way around. This way, if you have some fault in your car - you don't run the risk of frying the donor car unit.
There are a couple of fellas in the Syracuse area that I know of - try posting on the main rennlist board.
FWIW- my 87 runs fine with 59 psig fuel pressure. Given that most LH unit failures appear to start off as idle conrtol issues, and finally propagate to 'no' run conditions, I don't know that I'd suspect the LH unit at first - but could be. Tony had a similar problem on his 87 last summer. Turned out to be the LH unit after all. BTW - he had troubleshooted the MAF as well - with no avail.
Not much help I'm afraid - but please let us know what you find.
Last but not least - I know you don't want to hear this - but, invest in a set of manuals...they are lifesavers.
Best Regards-
Vasilis-
If you get the chance to check your LH unit - be certain to use your unit in the donor car - and not the other way around. This way, if you have some fault in your car - you don't run the risk of frying the donor car unit.
There are a couple of fellas in the Syracuse area that I know of - try posting on the main rennlist board.
FWIW- my 87 runs fine with 59 psig fuel pressure. Given that most LH unit failures appear to start off as idle conrtol issues, and finally propagate to 'no' run conditions, I don't know that I'd suspect the LH unit at first - but could be. Tony had a similar problem on his 87 last summer. Turned out to be the LH unit after all. BTW - he had troubleshooted the MAF as well - with no avail.
Not much help I'm afraid - but please let us know what you find.
Last but not least - I know you don't want to hear this - but, invest in a set of manuals...they are lifesavers.
Best Regards-
03-28-2002, 11:30 PM
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Hey guys
Thanks again for the advise and items to check...
DougM it sounds like to had the same problem with your cr in the past...And to answer Dr. Bob's question, the car is running at idle when the symptoms start to happen.
As Doug said, there is exhaust smoke (in my case white, not black) but the most puzzling thing is that the car stumbles and shakes badly when given any gas...
Steve C. I wish I could take you up on your offer but unfortunetely the car is not drivable right now...Thanks for the tips to check on the MAF sensor. I will try them over the weekend.
If the MAF sensor checks out okay, can I bring the LH brain over to you to test it in your car??
Let me know..
If you have any other tips on what to check, please feel free to contact me: 845-430-0456 cell
Vasilis
Thanks again for the advise and items to check...
DougM it sounds like to had the same problem with your cr in the past...And to answer Dr. Bob's question, the car is running at idle when the symptoms start to happen.
As Doug said, there is exhaust smoke (in my case white, not black) but the most puzzling thing is that the car stumbles and shakes badly when given any gas...
Steve C. I wish I could take you up on your offer but unfortunetely the car is not drivable right now...Thanks for the tips to check on the MAF sensor. I will try them over the weekend.
If the MAF sensor checks out okay, can I bring the LH brain over to you to test it in your car??
Let me know..
If you have any other tips on what to check, please feel free to contact me: 845-430-0456 cell
Vasilis
