jhend1000

Hi all, I recently purchased a 1989 Corvette to fix-up over the next few years. (My first 'Vette! ) I'm starting with the mechanical aspects first. My biggest issue is that when you try to shut the car off, it starts dieseling. I've seen this happen more on carbureted engines, but not with fuel injection. The previous owner told me that he has replaced the injectors twice, thinking that the issue would go away. He and I thought the timing might be a bit off but that wasn't the problem either. As a matter of fact, setting the timing at 6* before TDC like it should be causes the issue to be a little bit worse. Advancing the timing to somewhere closer to 10-12* TDC causes the engine to stop kind of roughly when you turn the key to the off position, but it doesn't diesel anymore. My guess is that it's running rich, because something is causing excess fuel to go into the cylinders. Seeing that the previous owner has replaced all 8 injectors twice makes me think that leaking injectors isn't the issue. What else could be causing the dieseling to occur?

antfarmer2

Do a pressure test and see how fast it drops. Pull the vacume line off the fpr and smell for fuel.

Matatk

On a fuel injected motor it would be caused by extra fuel after shutdown. So do a fuel pressure test to check for leaking injectors.

Cliff Harris

The fuel injectors are turned off when the ECM stops getting DRPs (Distributor Reference Pulses) from the ignition module inside the distributor. That should happen when the ignition switch is turned off and the power is removed from the distributor (pink wire). It sounds like the distributor is still getting powered somehow.

jhend1000

Cliff, this seems like it could be my issue. I'll definitely look into this some more.


Matak and antfarmer2, it's also possible that the previous owner improperly installed the injectors twice in a row. I'll try to check the fuel pressure when I get some free time. The car strongly smells of fuel vapor (like nostril burning strong) when it's running. I'd imagine it would be similar if I pull the vacuum from the FPR.


I'll keep everyone posted. Thanks for the input so far!

antfarmer2

A leaking fpr diaphram might do this put a pressure gauge on it and turn the key on without starting and see how long it holds

muzikmanwi

Have you reset your minimum idle speed?

jhend1000

No, I haven't messed with that yet. I don't think that's causing excess fuel to get into the cylinders. I will reset that when I've got it running properly, otherwise. I'm starting to now think it's something with the fuel system, rather than timing. I believe that advancing the timing just kind of corrects the issue. I'm starting to suspect that the FPR is acting up. I'll try to run the fuel pressure test this evening and I'll post results.

jhend1000

Ok, so I ran a fuel pressure test, and the pressure seems to hold pretty well. With key in on position and engine off, the pressure seems to hold fairly well staying between 39-41 psi. That was for roughly a minute. I turned the key to off and let the car sit for about 2 and a half minutes and the pressure dropped to around 35 psi. I started the engine and let it run for a bout 8 minutes. The pressure stayed constant at 33 psi. I didn't notice any visual signs of leaking around the injectors. I did smell gasoline in the FPR vacuum line, but there wasn't any fuel leaking or dripping from there either. Exhaust still strongly smells of fuel. This makes me think that the injectors are good. I'm guessing it's either the FPR, the ignition control module, or a bad sensor somewhere.


Edit: Here's something else I just noticed too: I've read that a bad coolant temperature sensor can cause a rich condition to occur. My coolant gauge on my digital cluster always reads "lo" even though the cooling system works fine, and the fan kicks in after the engine warms up.I had planned on replacing this sensor anyway in order to get an accurate reading on the temp. but I'm now thinking that this could be my main issue. I also spotted that the low-coolant sensor was unplugged. I'm guessing that when the coolant temp. sensor might have died, it threw off the low-coolant sensor and caused the low coolant light on the dash to illuminate. Plugging it back in, however didn't seem to change anything that I can tell. The low coolant light doesn't illuminate. (Could be a burnt out bulb.) I'll replace this sensor, and post back soon.

yd328

The coolant level sensors have been problematic and have nothing to do with your temp reading, could be why its disconnected. Also, the dash readout uses a different sensor than the ecm sensor. A bad ecm coolant sensor can cause a rich condition, if it is reading a low temp. Do you have access to a scanner? It would be helpful to diagnose this as you can read the coolant temp as the ecm sees it, along with other sensors and info.

Gary

aklim

A bad CTS can tell the ECM to run the motor richer because it is thinking it is cold out. However, I don't know if it can cause dieseling. Why don't you try this. Scan the temp around the sensor and use a scanner to see what the ECM thinks the temps are. It should be fairly close.

If Cliff Harris is right, how would that work that there are injector pulses? Maybe you can create a dieseling condition and put a noid light and see if it is really pulsing? If it isn't then fuel is somehow leaking into the combustion chambers. Still, what is igniting it?

antfarmer2

The digital temp sensor can do some strange things

aklim

There is one for the gauge and IIRC, one for the ECM? If so, WGAS what the gauge sees? We need to see what the ECM sees so if you did an IR scan of the sensor and it says 180 degrees and your ECM reads 30 degrees we can see a problem. Still what would keep the distributor powered?

jhend1000

True, that wouldn't keep the distributor going if that's the case here. If I borrow a scan tool from someone, along with an IR Temp. gun, I'll scan it and compare what the ECU thinks the temperature is to the actual temp. In the mean time, I think I'll look at the Ignition Control Module and see what's going on there. At the moment, with the timing advanced like it is now, it hasn't dieseled any when I turn the engine off. (But that's still not the correct timing! ). It's still running rich though and it's still got that strong scent when it's running. Any other thoughts as to what could be behind this?

antfarmer2

While your scanning see what the o2 is doing between that and the temp sensor you car does not know wether to shxt or wind it's watch

VikingTrad3r

dud u check fir faulty ruptered fpr yet? if its loading the vac lines with fuel that may br the source of fuel after shutdown.

antfarmer2

that was my first thought too. he seems to have alot going on.

aklim

But what is the source of ignition? Hot spots in the chamber?

Cliff Harris

The coolant temperature sensor that affects this is the one in the front of the intake manifold (or in the water pump on LT1 engines). This has no connection to the dash coolant temperature gauge.

Dieseling is caused by hot spots in the combustion chamber that ignite the fuel. The key here is that you have to have fuel in the combustion chamber, which shouldn't happen if everything is working correctly.

aklim

So lets assume he DOES have dieseling. This means he must have BOTH the hot spots AND fuel. Do you see any way the ECM would be having injector pulses? IF not, where is the fuel coming from? It must be a decent sized rupture in the diaphragm where the fuel is going into the intake manifold and that would be a fuel soaked line. That or the injectors are leaking fuel? So maybe he does have 2 problems? Leaking injectors AND hot spots? Fuel has to come from somewhere. Either ruptured diaphragm or leaking injectors. Not saying it is impossible but it would be hard to imagine 2 problems at once