1989 Corvette Dieseling Issue
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Joined: Feb 2016
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From: Gainesville Georgia
Iv'e eliminated the coolant sensor. I don't think that's my issue here. I repaired the problem with the gauge cluster not showing anything but "lo" by replacing the sending unit. That all works fine now. Today, I'm planning on replacing the distributor cap and rotor, as well as the ignition module, then resetting the timing. If that doesn't fix it, then I plan on taking the plenum off and investigating the fpr to see if it's ripped. I've read that the fpr cover has to be torqued to spec. for the fpr to operate correctly. Does anyone know if this is true? If so, what are the specs. to torque the screws down to?
Last edited by jhend1000; Mar 6, 2016 at 10:52 AM.
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From: Hartford WI
Iv'e eliminated the coolant sensor. I don't think that's my issue here. I repaired the problem with the gauge cluster not showing anything but "lo" by replacing the sending unit. That all works fine now. Today, I'm planning on replacing the distributor cap and rotor, as well as the ignition module, then resetting the timing. If that doesn't fix it, then I plan on taking the plenum off and investigating the fpr to see if it's ripped. I've read that the fpr cover has to be torqued to spec. for the fpr to operate correctly. Does anyone know if this is true? If so, what are the specs. to torque the screws down to?
If you want to see if the FPR is either ripped or not working right, you need to put a gauge on it. Turn on without running and it should instantly spike to about 43psi. Same idling with the FPR hose off. If you do a WOT run, it might dip down a bit but should hit steady 40s during the run.
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From: Gainesville Georgia
Okay, so I swapped the ignition module, cap, and rotor, and that made the car run a little bit smoother but that's about it. I've also now noticed that the dieseling issue is very intermittent. Running another fuel pressure test today made me realize that sometimes, the fuel pressure is close to what it should be, and other times, like earlier today, it is below what it should be. Here are my results from earlier today:
- With key on and engine off: slowly built up to 30 PSI
- With engine running and FPR vacuum line on, steady at 31 PSI
- With FPR vacuum line off: 41 PSI
- With key on and engine off: slowly built up to 30 PSI
- With engine running and FPR vacuum line on, steady at 31 PSI
- With FPR vacuum line off: 41 PSI
Last edited by jhend1000; Mar 7, 2016 at 09:36 PM.
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Once pressure is up how fast does it bleed down? What is your fuel pressure suppose to be mine is around 40.. that sounds low how old is your fuel filter? Might be a weak pump or leaky pulsater.
Last edited by antfarmer2; Mar 7, 2016 at 04:02 PM.
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From: Hartford WI
Okay, so I swapped the ignition module, cap, and rotor, and that made the car run a little bit smoother but that's about it. I've also now noticed that the dieseling issue is very intermittent. Running another fuel pressure test today made me realize that sometimes, the fuel pressure is close to what it should be, and other times, like earlier today, it is below what it should be. Here are my results from earlier today:
- With key on and engine off: slowly built up to 30 PSI
- With engine running and RPR vacuum line on, steady at 31 PSI
- With FPR vacuum line off: 41 PSI
- With key on and engine off: slowly built up to 30 PSI
- With engine running and RPR vacuum line on, steady at 31 PSI
- With FPR vacuum line off: 41 PSI
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Joined: Feb 2016
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From: Gainesville Georgia
That is weird. How good is your fuel filter? I would say the next thing is the pump since that is what builds pressure up. Have you checked it lately? Either the filter is hopelessly clogged or something is wrong/ Running at 41 with the line off simulates a WOT. It might be that you have to go to the pump area and see if the fuel pulsator is leaking and working against the pump trying to gather pressure.
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if we are going with the theory that the dieseling is caused from fuel being delivered to the cylinder after the pump and injectors have been shut down, that fuel has to stored up somewhere that gets back tobtge intake.
fuel pressure regulator letting fuel into vac hoses or maybe the charcoal cannister is full of gas and lets it come in the vac line up past the pcv intake inlet.
furthermore, if the dizzy is spinning, are the plugs firing?
fuel pressure regulator letting fuel into vac hoses or maybe the charcoal cannister is full of gas and lets it come in the vac line up past the pcv intake inlet.
furthermore, if the dizzy is spinning, are the plugs firing?
I haven't checked the fuel filter, fuel pump, and pulsator out yet. The previous owner gave me a new fuel filter he bought but never installed on it. Maybe this is why... However, I don't understand how that causes a rich condition to occur. Could someone please explain it to me?
Last edited by VikingTrad3r; Mar 7, 2016 at 07:59 PM.
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From: Hartford WI
I haven't checked the fuel filter, fuel pump, and pulsator out yet. The previous owner gave me a new fuel filter he bought but never installed on it. Maybe this is why... However, I don't understand how that causes a rich condition to occur. Could someone please explain it to me?
I had a car that ran pig rich. Yes, we smell it too. OTOH, the ECM said it was running 0V which is extremely lean. We got the fuel pressure right, checked the O2 sensor and even found that the driver side was glowing red. Long story short, there was a short in the injector wiring and the driver side shorted out and that was the bank the O2 sensor was reading. As such, the ECM decided the ENTIRE engine was lean so it dumped excess fuel into the system but only the right side injectors were firing and drowning the spark out.
Once we establish you have good spark and good fuel pressure, we can start going for more exotic solutions.
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From: Gainesville Georgia
Unfortunately, replacing the fuel pressure regulator didn't fix the issue. It ran about the same. On the bright side, along with the fpr, I replaced the plenum and throttle body gaskets for good measure. I haven't seen any fuel leaking around the charcoal canister, but I'll investigate it further. I may pick up a fuel pump in the next few days based on what else I find. I'm still thinking that the est might play a role here because I recall that the car ran better after disconnecting the est plug and setting the timing. After plugging it back in, and starting it up later, the timing somehow advanced itself by several degrees.
Last edited by jhend1000; Mar 11, 2016 at 10:24 PM.
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From: Gainesville Georgia
I haven't checked for that but I will when I get off of work later. The car was missing the dipstick when I purchased it. I bought a new dipstick for it now. When I did check the oil, it looked ok and the viscosity didn't seem off either. I have a new theory, if he fuel pump is weak or the pulsator is bad, could the ECM be trying to compensate for the lack of fuel from the bad pump? Perhaps it's seeing it in a way that it causes the injectors to dump too much fuel into the cylinders. Is it normal for the EST to change the timing way past 6 degrees after it's plugged back in?
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From: Hartford WI
Still, it doesn't answer the question. You have air and fuel. What is the spark coming from to cause dieseling?
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I would change the oil.
Last edited by antfarmer2; Mar 12, 2016 at 02:09 PM.
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From: Hartford WI
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Joined: Feb 2016
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From: Gainesville Georgia
The previous owner told me that he changed the oil roughly a week before I bought the car. I've had the car for almost 3 weeks now I think. I'll still look at the oil anyway.