1979 warm start almost impossible help pleaaase :(
#31
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Definitely leaking injectors.
Had exact same problem with my previous '79.
When trying to restart afte 20 mins or so, the injectors have been leaking fuel into the cylinders, causing a flooding condition.
By the time the engine cools down, the fuel pressure in the lines is at zero, the injectors have stopped leaking and the fuel in the cylinders has evaporated so it then starts fine.
Had exact same problem with my previous '79.
When trying to restart afte 20 mins or so, the injectors have been leaking fuel into the cylinders, causing a flooding condition.
By the time the engine cools down, the fuel pressure in the lines is at zero, the injectors have stopped leaking and the fuel in the cylinders has evaporated so it then starts fine.
CIS is wonderful when it's used and driven regularly so it stays clean. Because it depends on careful balance of fuel and control pressures, and is sensitive to blind changes and adjustments, dirt and fiddly folks are usually the biggest enemies of good CIS.
#32
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Thank's a lot Dr Bob and Randy. I'm waiting for the check valve to put on fuel line I ordered to continue working on my car. To check the plugs after the crucial non starting period is an excellent idea.
Should I put injectors cleaner to help lower the chances of having problems since I winterize my 928 for several months?
Another theorical question and forgive my ignorance: Since the injectors are not electrically powered on CIS cars, what makes them inject at the right time?
Should I put injectors cleaner to help lower the chances of having problems since I winterize my 928 for several months?
Another theorical question and forgive my ignorance: Since the injectors are not electrically powered on CIS cars, what makes them inject at the right time?
Last edited by am; 05-20-2013 at 07:49 PM.
#33
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The fuel distributor can also be a source of trouble as it is a delicate piece of equipment that can be impacted by dirty fuel clogging the small screens located in each of the 8 distributor ports.
I recommend you do a google search for CIS fuel system overview. You need to understand how the overall system functions in order to troubleshoot and repair it.
#34
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The C in CIS is for continuous.
Once the pressure in the line to each injector rises about 45psi (I think), they start spraying.
The fuel distributor is one big variable pressure regulator. The more fuel you need, the higher it raises the pressure in the lines to the injectors.
Once the pressure in the line to each injector rises about 45psi (I think), they start spraying.
The fuel distributor is one big variable pressure regulator. The more fuel you need, the higher it raises the pressure in the lines to the injectors.
#35
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Thank's for excellent explanations. I know the next question is not related to my problem but by curiosity... What synchronise the CIS to pressurise the right injector at the right time?
#36
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The 928 electronic fuel injection (L-Jetronic and LH-Jetronic) is a batch fire system, all eight injectors fire at the same time. More modern injection systems provide sequential injection where injectors opening is synchronized to the intake stroke of each cylinder .... but that was done not on the Porsche 928.
#38
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yes, any injector could leak when the motor is off. However, when the motor is running, CIS literally means continuous injection system - so in essence they are always 'leaking'. Generally, CIS give bad spray patterns when old or clogged which is more around poor performance than anything.
#39
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lol - i just realized RICH be me to the punch!
#41
Former Vendor
Trying to fix CIS injection without a pressure gauge is like trying to figure out what color something is, without your eyes open.
"Your" problem is a very typical CIS problem.....and one of the first questions I ask, when someone is trying to tell me what their CIS vehicle is doing wrong.
"So, if you get your car warm and go into the grocery store for 15 minutes....does the car re-start?"
Of course, the answer is usually "No".
99.9% of the time, this means there is a pressure leak.....and that pressure leak can happen anywhere. Fuel pump check valve, fuel accumulator, injectors, fuel distributor, warm-up regulator, cold start valve, leaky fuel line.
The "system" must retain a certain fuel pressure for a certain amount of time. This exact specification is in the literature, but is about 1.5 bar for 20 minutes. Yours probably drops to 0 bar, in 20 seconds.
What is happening is the fuel that is in the system is being forced, from the engine heat, back to the tank. The heat in the fuel lines then turns the remaining fuel into vapor. The result is like running out of fuel and having vapor lock at the same instance....the car must be cranked a very long time....or cool down to re-start.
Without a pressure gauge, you can only replace one component at a time, until you fix the car. With a pressure gauge, you can isolate which component is bad, using a couple of fuel line clamps.
For instance, to test the fuel pump check valve, you install the gauge and monitor fuel pressure. Have someone shut off the car, the same instant that you "clamp" the fuel feed line shut. If the pressure holds....you have a bad check valve....if the pressure still drops, the problem is somewhere else.
You can do the same test for the accumulator....simply crimp off the fuel line between the engine and the accumulator. If the pressure still drops....you have a problem somewhere else. If the pressure holds, you have a bad accumulator (or a bad fuel pump check valve, which hopefully you tested first.)
Warm-up regulator or fuel distributor leaking? Crimp off the return line.
Injectors or cold start leaking? Crimp off both the return line and the feed line at the same instant.
With a little bit of logic and a pressure gauge, you can figure this out pretty easily. Without either...you are going to "throw money" at the vehicle until you get lucky.
"Your" problem is a very typical CIS problem.....and one of the first questions I ask, when someone is trying to tell me what their CIS vehicle is doing wrong.
"So, if you get your car warm and go into the grocery store for 15 minutes....does the car re-start?"
Of course, the answer is usually "No".
99.9% of the time, this means there is a pressure leak.....and that pressure leak can happen anywhere. Fuel pump check valve, fuel accumulator, injectors, fuel distributor, warm-up regulator, cold start valve, leaky fuel line.
The "system" must retain a certain fuel pressure for a certain amount of time. This exact specification is in the literature, but is about 1.5 bar for 20 minutes. Yours probably drops to 0 bar, in 20 seconds.
What is happening is the fuel that is in the system is being forced, from the engine heat, back to the tank. The heat in the fuel lines then turns the remaining fuel into vapor. The result is like running out of fuel and having vapor lock at the same instance....the car must be cranked a very long time....or cool down to re-start.
Without a pressure gauge, you can only replace one component at a time, until you fix the car. With a pressure gauge, you can isolate which component is bad, using a couple of fuel line clamps.
For instance, to test the fuel pump check valve, you install the gauge and monitor fuel pressure. Have someone shut off the car, the same instant that you "clamp" the fuel feed line shut. If the pressure holds....you have a bad check valve....if the pressure still drops, the problem is somewhere else.
You can do the same test for the accumulator....simply crimp off the fuel line between the engine and the accumulator. If the pressure still drops....you have a problem somewhere else. If the pressure holds, you have a bad accumulator (or a bad fuel pump check valve, which hopefully you tested first.)
Warm-up regulator or fuel distributor leaking? Crimp off the return line.
Injectors or cold start leaking? Crimp off both the return line and the feed line at the same instant.
With a little bit of logic and a pressure gauge, you can figure this out pretty easily. Without either...you are going to "throw money" at the vehicle until you get lucky.
Last edited by GregBBRD; 05-21-2013 at 10:34 PM.
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#42
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At this point I'm trying to understand perfectly how the system works. This forum is better than many books because of the great experience and knowledge of the persons who want to help by writing back.
I'll get the gauges and books it takes for the trouble shooting in time.
I'll get the gauges and books it takes for the trouble shooting in time.
#44
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am- in our most polite way ... You must have the book and gauges. These tell you exactly where the problem lies. Again, think of calling a surgeon to diagnose heart problems over the phone. Without a blood pressure test, etc he has no baseline. Same thing!!!! We need baseline numbers !!! How many more of these!!!!!!!!!!! Do i need to put down...
#45
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The Watson book is excellent, but hard to find cheap now. get it or similar, and the gauge set (Roger T does a good one), and make systematic tests. I had similar hot start issues once. Fixed for some time with a new accumulator, then the half hot start temp sensor that fires a solenoid failed - I don't think your model had this part. Pressure testing will tell you a Lot!
jp 83 Euro S AT 55k
jp 83 Euro S AT 55k