944-starts, stalls, backfires/pops
05-14-2018, 10:04 AM
#1
2nd Gear
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944-starts, stalls, backfires/pops
Hi all,
First post here, so I hope this is the correct place for this sort of post. Thanks in advance for the help. The car is 1984 944 non turbo. I recently purchased the car and it initially had a hard start issue. The previous owner installed a new fuel pump, but wired it directly to the fuse panel. You can hear the pump cut on with the key in the on position, and this repair seems to work fine for time being. I tested fuel pressure etc. (see below). If I wanted the car to start I would have to give it a bit of throttle to keep it running. After holding the throttle for about 10 to 30 seconds the car would idle and run rough. By run rough I mean it acted as if the idle was too low and it wanted to stall. At that point I inspected and tested the following:
fuel pressure test. Pressure with key on, starting, running and leakdown were within spec.
inspected the vacuum lines. They were dry rotted. I pulled the intake manifold and replaced them all with new rubber.
While I had the intake off I tested the following:
DME temp sensor-it ohm'd out ok
Bench tested- MAF sensor using Clark's garage method-it may have a small dead spot but not where it would point at idle
I also tested the Idle control valve. It has the old style. While i could not see the shutter/knife valve slowly open and close, when 12v was applied it did have continuity etc.
After reassembling everything I went to start the car, and fired up with no pedal, but shuts off within 2 to 3 seconds. When it shuts off I hear a small pop/possible back fire. Applying pedal has no impact. At that point, I tested/checked the following:
- Triple checked the firing order. It is 100 % correct.
- Triple checked vacuum lines and there routing. Is correct as well. No kinks, leak points etc.
- I preformed an ohm test on the speed and reference sensors. Both tested at 1070 ohms, which is in spec.
- Checked the vacuum line running to the throttle body, and switched them. No difference.
- unplugged MAF sensor. No difference.
At this point I am at a loss and don't know where to start checking with this thing. What is strange is that problem changed after reassembly. Before the vacuum lines where redone the car would not start without some pedal applied, and afterwards it will start and run for a brief 2 to 3 seconds, but application of the throttle as makes no impact. Anyone have any ideas about what to check next, or how to attack this thing?
First post here, so I hope this is the correct place for this sort of post. Thanks in advance for the help. The car is 1984 944 non turbo. I recently purchased the car and it initially had a hard start issue. The previous owner installed a new fuel pump, but wired it directly to the fuse panel. You can hear the pump cut on with the key in the on position, and this repair seems to work fine for time being. I tested fuel pressure etc. (see below). If I wanted the car to start I would have to give it a bit of throttle to keep it running. After holding the throttle for about 10 to 30 seconds the car would idle and run rough. By run rough I mean it acted as if the idle was too low and it wanted to stall. At that point I inspected and tested the following:
fuel pressure test. Pressure with key on, starting, running and leakdown were within spec.
inspected the vacuum lines. They were dry rotted. I pulled the intake manifold and replaced them all with new rubber.
While I had the intake off I tested the following:
DME temp sensor-it ohm'd out ok
Bench tested- MAF sensor using Clark's garage method-it may have a small dead spot but not where it would point at idle
I also tested the Idle control valve. It has the old style. While i could not see the shutter/knife valve slowly open and close, when 12v was applied it did have continuity etc.
After reassembling everything I went to start the car, and fired up with no pedal, but shuts off within 2 to 3 seconds. When it shuts off I hear a small pop/possible back fire. Applying pedal has no impact. At that point, I tested/checked the following:
- Triple checked the firing order. It is 100 % correct.
- Triple checked vacuum lines and there routing. Is correct as well. No kinks, leak points etc.
- I preformed an ohm test on the speed and reference sensors. Both tested at 1070 ohms, which is in spec.
- Checked the vacuum line running to the throttle body, and switched them. No difference.
- unplugged MAF sensor. No difference.
At this point I am at a loss and don't know where to start checking with this thing. What is strange is that problem changed after reassembly. Before the vacuum lines where redone the car would not start without some pedal applied, and afterwards it will start and run for a brief 2 to 3 seconds, but application of the throttle as makes no impact. Anyone have any ideas about what to check next, or how to attack this thing?
05-14-2018, 11:33 AM
#2
Three Wheelin'
Hi all,
First post here, so I hope this is the correct place for this sort of post. Thanks in advance for the help. The car is 1984 944 non turbo. I recently purchased the car and it initially had a hard start issue. The previous owner installed a new fuel pump, but wired it directly to the fuse panel. You can hear the pump cut on with the key in the on position, and this repair seems to work fine for time being. I tested fuel pressure etc. (see below). If I wanted the car to start I would have to give it a bit of throttle to keep it running. After holding the throttle for about 10 to 30 seconds the car would idle and run rough. By run rough I mean it acted as if the idle was too low and it wanted to stall. At that point I inspected and tested the following:
fuel pressure test. Pressure with key on, starting, running and leakdown were within spec.
inspected the vacuum lines. They were dry rotted. I pulled the intake manifold and replaced them all with new rubber.
While I had the intake off I tested the following:
DME temp sensor-it ohm'd out ok
Bench tested- MAF sensor using Clark's garage method-it may have a small dead spot but not where it would point at idle
I also tested the Idle control valve. It has the old style. While i could not see the shutter/knife valve slowly open and close, when 12v was applied it did have continuity etc.
After reassembling everything I went to start the car, and fired up with no pedal, but shuts off within 2 to 3 seconds. When it shuts off I hear a small pop/possible back fire. Applying pedal has no impact. At that point, I tested/checked the following:
- Triple checked the firing order. It is 100 % correct.
- Triple checked vacuum lines and there routing. Is correct as well. No kinks, leak points etc.
- I preformed an ohm test on the speed and reference sensors. Both tested at 1070 ohms, which is in spec.
- Checked the vacuum line running to the throttle body, and switched them. No difference.
- unplugged MAF sensor. No difference.
At this point I am at a loss and don't know where to start checking with this thing. What is strange is that problem changed after reassembly. Before the vacuum lines where redone the car would not start without some pedal applied, and afterwards it will start and run for a brief 2 to 3 seconds, but application of the throttle as makes no impact. Anyone have any ideas about what to check next, or how to attack this thing?
First post here, so I hope this is the correct place for this sort of post. Thanks in advance for the help. The car is 1984 944 non turbo. I recently purchased the car and it initially had a hard start issue. The previous owner installed a new fuel pump, but wired it directly to the fuse panel. You can hear the pump cut on with the key in the on position, and this repair seems to work fine for time being. I tested fuel pressure etc. (see below). If I wanted the car to start I would have to give it a bit of throttle to keep it running. After holding the throttle for about 10 to 30 seconds the car would idle and run rough. By run rough I mean it acted as if the idle was too low and it wanted to stall. At that point I inspected and tested the following:
fuel pressure test. Pressure with key on, starting, running and leakdown were within spec.
inspected the vacuum lines. They were dry rotted. I pulled the intake manifold and replaced them all with new rubber.
While I had the intake off I tested the following:
DME temp sensor-it ohm'd out ok
Bench tested- MAF sensor using Clark's garage method-it may have a small dead spot but not where it would point at idle
I also tested the Idle control valve. It has the old style. While i could not see the shutter/knife valve slowly open and close, when 12v was applied it did have continuity etc.
After reassembling everything I went to start the car, and fired up with no pedal, but shuts off within 2 to 3 seconds. When it shuts off I hear a small pop/possible back fire. Applying pedal has no impact. At that point, I tested/checked the following:
- Triple checked the firing order. It is 100 % correct.
- Triple checked vacuum lines and there routing. Is correct as well. No kinks, leak points etc.
- I preformed an ohm test on the speed and reference sensors. Both tested at 1070 ohms, which is in spec.
- Checked the vacuum line running to the throttle body, and switched them. No difference.
- unplugged MAF sensor. No difference.
At this point I am at a loss and don't know where to start checking with this thing. What is strange is that problem changed after reassembly. Before the vacuum lines where redone the car would not start without some pedal applied, and afterwards it will start and run for a brief 2 to 3 seconds, but application of the throttle as makes no impact. Anyone have any ideas about what to check next, or how to attack this thing?
05-14-2018, 11:51 AM
#3
Rennlist Member
My guess would be some sort of vacuum leak that is downstream of the air meter, thus causing the mixture to be too lean.
I also have an 84. A global check, if you have a shop vac or some shop air, is to make up a fitting that goes over the inlet to the J hose and pressurize the inlet track while listening for leaks. Harder than it sounds, since you will always be pushing some air out of an open intake or exhaust valve. But will identify large holes! Take your Jboot off and flex it back and forth while giving it a visual inspection. In can crack down in the accordion folds.
A test that I have done is to remove all the "large" (~1") hoses that attach to the manifold and intake J boot and plug the entry to the manifold. When you do this, you take a lot of potential leak paths out of play. You are also taking the idle air bypass gizmo out of service and the brake booster. Of the small lines (~1/4"), there is one just upstream of the throttle plate and one downstream of the throttle plate. The one upstream can also be plugged. It's associated with gas tank vapor recovery system.
This leaves the one downstream of the throttle plate. This vacuum line goes to a tee fitting that branches to your fuel pressure regulator and the fuel pressure damper. Poorly understood is that your static fuel pressure (~44 psi) gets reduced to 36-40 psi at idle. The idle vacuum applied to the FPR does this. If this is not happening, you have excess pressure at the injectors and the idle mixture will be too rich. The FPR is the one that exits to the return line to the tank. On the other hand, I don't think the other one, the pressure damper, is mission critical. Both of these of course can have in internal failure and just be sucking in room air, which is clearly not good. You can give this a modest test simply by hooking up a bit of clean hose to them and sucking on them. If they are totally shot, you should be able to spot it. Better is to put a MityVac on them and see if they will hold normal vacuum i.e. ~15-20" water. So if you gotten that far, and if you don't already have one, get a cheap vacuum gauge. Tee it into the line to the FPR/FPD and if you can get the car to start, ensure that the vacuum in the line is "reasonable" i.e. 15-20" water. A worn out engine will be on the low end but the point is the number shouldn't be like 0, 5, or 10. And better, but more expensive, is to get a fuel pressure gauge and mount it on the end of the fuel rail and verify that the fuel pressure is dropping from 44 psi to 36-40 psi at idle.
If you can get the car to idle in this test configuration (it's OK to open up the idle air bypass screw on the throttle body just to keep it running), then work backward introducing the various small and large hoses. If they are original to the car, I would be shocked if you don't find some with cracks. And/or you will find "kludges" made up from non-OEM hoses. Personally, I just bite the bullet and buy all new OEM hoses from Paragon but I know that can be quite the investment.
There is a chance that you have a vacuum leak in the brake booster. You can just compress the hose to the booster with some pliers or vice grips and see if that helps.
If you haven't already, clean up your throttle body and make sure the throttle plate closes completely. Not mission critical but make sure you hear the microswitch open/close as the throttle plate goes fully closed. Make sure the oring on the idle bypass screw is not shot. Make sure that small passages leading the vacuum ports are open.
If the engine can be made to run, even poorly, some people introduce a volatile solvent such as starting fluid locally at suspect hose locations. If the engine speeds up, there is a vacuum leak that has sucked in the starting fluid.
I never say never but if you can get the car to start and idle at all, I would rule out electrical items like the two crank sensors, the DME, timing, etc. Although I have had cars that idled badly and when I looked in the engine compartment at night the sparks were flying from failed spark plug leads, etc. If you can keep it running you can do things like pull one spark plug lead at a time and see if the engine notices. Ditto for the fuel injector wiring connectors.
If we get really desperate, we're going to pull the fuel injector rail and test fire the injectors into cups but let's hold off on that for now.
Good luck, keep us posted. Think how good you'll feel when you get this resolved!
I also have an 84. A global check, if you have a shop vac or some shop air, is to make up a fitting that goes over the inlet to the J hose and pressurize the inlet track while listening for leaks. Harder than it sounds, since you will always be pushing some air out of an open intake or exhaust valve. But will identify large holes! Take your Jboot off and flex it back and forth while giving it a visual inspection. In can crack down in the accordion folds.
A test that I have done is to remove all the "large" (~1") hoses that attach to the manifold and intake J boot and plug the entry to the manifold. When you do this, you take a lot of potential leak paths out of play. You are also taking the idle air bypass gizmo out of service and the brake booster. Of the small lines (~1/4"), there is one just upstream of the throttle plate and one downstream of the throttle plate. The one upstream can also be plugged. It's associated with gas tank vapor recovery system.
This leaves the one downstream of the throttle plate. This vacuum line goes to a tee fitting that branches to your fuel pressure regulator and the fuel pressure damper. Poorly understood is that your static fuel pressure (~44 psi) gets reduced to 36-40 psi at idle. The idle vacuum applied to the FPR does this. If this is not happening, you have excess pressure at the injectors and the idle mixture will be too rich. The FPR is the one that exits to the return line to the tank. On the other hand, I don't think the other one, the pressure damper, is mission critical. Both of these of course can have in internal failure and just be sucking in room air, which is clearly not good. You can give this a modest test simply by hooking up a bit of clean hose to them and sucking on them. If they are totally shot, you should be able to spot it. Better is to put a MityVac on them and see if they will hold normal vacuum i.e. ~15-20" water. So if you gotten that far, and if you don't already have one, get a cheap vacuum gauge. Tee it into the line to the FPR/FPD and if you can get the car to start, ensure that the vacuum in the line is "reasonable" i.e. 15-20" water. A worn out engine will be on the low end but the point is the number shouldn't be like 0, 5, or 10. And better, but more expensive, is to get a fuel pressure gauge and mount it on the end of the fuel rail and verify that the fuel pressure is dropping from 44 psi to 36-40 psi at idle.
If you can get the car to idle in this test configuration (it's OK to open up the idle air bypass screw on the throttle body just to keep it running), then work backward introducing the various small and large hoses. If they are original to the car, I would be shocked if you don't find some with cracks. And/or you will find "kludges" made up from non-OEM hoses. Personally, I just bite the bullet and buy all new OEM hoses from Paragon but I know that can be quite the investment.
There is a chance that you have a vacuum leak in the brake booster. You can just compress the hose to the booster with some pliers or vice grips and see if that helps.
If you haven't already, clean up your throttle body and make sure the throttle plate closes completely. Not mission critical but make sure you hear the microswitch open/close as the throttle plate goes fully closed. Make sure the oring on the idle bypass screw is not shot. Make sure that small passages leading the vacuum ports are open.
If the engine can be made to run, even poorly, some people introduce a volatile solvent such as starting fluid locally at suspect hose locations. If the engine speeds up, there is a vacuum leak that has sucked in the starting fluid.
I never say never but if you can get the car to start and idle at all, I would rule out electrical items like the two crank sensors, the DME, timing, etc. Although I have had cars that idled badly and when I looked in the engine compartment at night the sparks were flying from failed spark plug leads, etc. If you can keep it running you can do things like pull one spark plug lead at a time and see if the engine notices. Ditto for the fuel injector wiring connectors.
If we get really desperate, we're going to pull the fuel injector rail and test fire the injectors into cups but let's hold off on that for now.
Good luck, keep us posted. Think how good you'll feel when you get this resolved!
05-14-2018, 12:32 PM
#4
Rennlist Member
Did you mess with the injectors at all? They might be clogged if you didn't. They might have the wrong kit if you did. My car did pretty much what you're describing after I removed my intake manifold and redid the vacuum lines, and it was all the injectors.
See my experience here: https://rennlist.com/forums/924-931-...experts-2.html
See my experience here: https://rennlist.com/forums/924-931-...experts-2.html
05-14-2018, 01:01 PM
#5
2nd Gear
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Divil- It is strange that plugging in or unplugging the AFM makes no difference. I would think that unplugging the AFM would result in the computer returning to default setting and have some sort of impact. I will check the AFM leads tonight for power/ground etc.
Harveyf- Thanks for the all the suggestion. The J-boot (intake hose from the filter to the throttle body) is new. It disintegrated when i took it off. I also cleaned out the throttle body (with CRC throttle body cleaner) as it needed a good cleaning when I took it apart. I will try plugging off the manifold ports and the upstream throttle body vacuum line to see if that makes a difference. I will proceed with the rest of the tests from there. Right now the car won't run long enough to do some of the other tests.
Ashman- When I pulled the fuel rail I replaced all the o-rings, as well as a pintle tip on one injector (with a blue pintle tip). The 3 other injectors have the stock pintle tips on them. However, when I could keep the car running before replacing vacuum lines etc the injectors must have been working ( if only marginally to keep it running) and they must be firing. What I don't understand is why they would fire temporarily... or not be delivering enough fuel.
Anyhow, thanks everyone for the ideas and keep them coming. I will report back once I run some of the tests.
Harveyf- Thanks for the all the suggestion. The J-boot (intake hose from the filter to the throttle body) is new. It disintegrated when i took it off. I also cleaned out the throttle body (with CRC throttle body cleaner) as it needed a good cleaning when I took it apart. I will try plugging off the manifold ports and the upstream throttle body vacuum line to see if that makes a difference. I will proceed with the rest of the tests from there. Right now the car won't run long enough to do some of the other tests.
Ashman- When I pulled the fuel rail I replaced all the o-rings, as well as a pintle tip on one injector (with a blue pintle tip). The 3 other injectors have the stock pintle tips on them. However, when I could keep the car running before replacing vacuum lines etc the injectors must have been working ( if only marginally to keep it running) and they must be firing. What I don't understand is why they would fire temporarily... or not be delivering enough fuel.
Anyhow, thanks everyone for the ideas and keep them coming. I will report back once I run some of the tests.
05-14-2018, 02:24 PM
#6
Official Bay Area Patriot
Fuse 24 Assassin
Rennlist Member
Fuse 24 Assassin
Rennlist Member
If there's a local Rennlister near you, swap AFMs with him ('83-early 85). If it runs better, replace the AFM. Identical issue my old 1986 had and I replaced the AFM. All was well after.
The fact you can unplug the AFM and it makes no difference points you in that direction. However, are you sure there are no cracks in the intake boot, or where the auxiliary air valve connects to it?
Good luck.
The fact you can unplug the AFM and it makes no difference points you in that direction. However, are you sure there are no cracks in the intake boot, or where the auxiliary air valve connects to it?
Good luck.
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05-14-2018, 05:00 PM
#9
Official Bay Area Patriot
Fuse 24 Assassin
Rennlist Member
Fuse 24 Assassin
Rennlist Member
05-16-2018, 03:10 PM
#10
If you do figure this out, let us know what it was. I had kind of the same problem..
Afm tested ok
Fuel pressure at the rail
Injectors firing
Dme engine temp tested ok,
Had tach bounce
No vacuum leaks on pressure tested intake
repaired dme
Car would start hard cold, fire once, quit, back fire. Then fire again and run just enough to drive around a bit.. But once it got hot and you shut it off it would just crank and crank and never fire up.
I ended up removing the afm for a map sensor, new wiring harness, and aftermarket ecu. No problems now except I haven't finished the fine tune.
Afm tested ok
Fuel pressure at the rail
Injectors firing
Dme engine temp tested ok,
Had tach bounce
No vacuum leaks on pressure tested intake
repaired dme
Car would start hard cold, fire once, quit, back fire. Then fire again and run just enough to drive around a bit.. But once it got hot and you shut it off it would just crank and crank and never fire up.
I ended up removing the afm for a map sensor, new wiring harness, and aftermarket ecu. No problems now except I haven't finished the fine tune.
05-16-2018, 03:24 PM
#11
Official Bay Area Patriot
Fuse 24 Assassin
Rennlist Member
Fuse 24 Assassin
Rennlist Member
Just curious, what does that ground look like for your DME? Cleaning those to a shiny copper can improve many running issues. Us 928 guys struggle with these grounds to our LH and EZK ECUs. Once they are mirror clean, running issues are a thing of the past. These old Porsches can be quite finicky with grounds.
