While researching why my C3 started dieseling, I came across an old threat discussing EGR valves on C3s. Did C3s have EGR valves or did the people participating in this thread confuse EGR with PCV valves?

 

C3's had both. Different function.
Check your timing.

 

KenSny, thanks for your fast response. I did not know they had both. I will check mine as it is listed as a possible cause of dieseling. I use the right fuel, had the timing checked, the fuel/air mixture and idle speed are good, and will check the spark plugs and for possible vacuum leaks next.

 

Timing or too rich carb setting, or too high idle
PVC should have no effect except for causing oil leaks
EGR usually causes stumbling and rough idle

 

If you are running ported Vacuum to the Dissy, switch to manifold vacuum and re-adjust the idle .

 

GM cars starting using EGR in 1973, so any C3 Vette built 1973 and after will have the EGR. PCV has been used on all GM cars since the early 1960s. The two systems have nothing in common - they serve completely different functions, so they are both used together. The EGR or PCV will not cause, or contribute to, run-on (aka, "dieseling") after shutdown, since the EGR is closed and inoperable at idle. The EGR only opens during light throttle cruise based on ported vacuum signal and engine temperature. Run-on after shutdown is caused by the throttle plates being open too far at shutdown, which can be caused by improper tuning and/or retarded timing. If setting your timing up correctly (adequate initial and vacuum advance) does not correct the problem, install and use the anti-dieseling idle-up solenoid that was used by GM to solve the problem.

See my post #4 here:
https://www.corvetteforum.com/forums...egr-valve.html


Lars

 

Lars, in regards to your comment about the GM anti-dieseling solenoid, I think you might be interested in this post from another thread.

I have a Holley anti-dieseling solenoid installed with a new Holley carburettor and I am noticing that if I rev to 5500 rpm in neutral and take my foot off the accelerator the rpm drops and the car stalls. As soon as my foot is off the rpm drops to 300-400 rpm and the car tries to recover but does not.The idle speed is approximately 550 rpm and the throttle position is set by the solenoid and the idle mixture.If I blip the throttle to say 4000 rpm, the engine drops to about 450 and bounces up to idle and settles, seems to be a wide open throttle issue.Is the issue likely that when the throttle is closed it is bumping the solenoid actuator resulting in complete closure of the throttle or a simple case of insufficient fuel and the idle needs to be bumped up using the mixture screws (and not adjusting how far the solenoid extends) ?Thanks,NJ

There were several responses to this post.

 

Blue1972, I did some research and it appears your recommendation regarding switching to manifold vacuum is valid, and I will do it. Subsequent to that, I will also have my timing checked again and redo my fuel/air mix and idle setting. Thanks for the information

 

Lars, my C3 had a crate ZZ4 engine when I purchased it, and since it does not have a EGR, I assume it was removed when installing the ZZ4 engine. I though I had posted this response earlier, but I don't see it. Thanks for clarifying the differences between the EGR and PCV valves.

 

The ZZ4 crate engine is not supplied with an EGR, since GM does not know what year car or application you're going to use the engine in. The ZZ4 intake has provision for an EGR to be installed, if the application/year requires one. So the EGR was never "removed" from your ZZ4 - the engine was not supplied with one, yet has the provision for it.

Not sure what the reference was about the guy using an improperly installed or functioning Holley idle solenoid, and doing stupid things like rev'ing the engine to 5500 rpm in neutral (WTF??). A correctly installed and operable solenoid works perfectly for solving run-on issues if you can't solve the problem with tuning and timing. GM used them for many years - there is no issue with a solenoid, if it is installed correctly and functioning as it should. If you're running a Holley-based carb, I'd suggest you take a look at how much transition slot exposure you have on the primary side of the carb - if you have more than .020" you have a carb setup issue and/or timing problems that are causing your run-on.

Lars

 

Thanks again for the information on the ZZ4. I adjusted the fuel/air mixture again with a vacuum gage. I hope this fixes my dieseling problem. When I was done with the adjustment the idle went from 6 to 9,000. I reduced it to 6,000 with the idle screw.

 

first I think you added an extra 0 to those numbers.
Did you recheck the mixture adjustment after dropping the idle speed to 600 rpm?

 

Yes, I got a little ambitious with the 0s. I've never had that engine at 6,000 much less 9,000 RPMs. No, as I stated I did the fuel/air mixture first resulting in idle increasing from 600 to 900. I then adjusted the idle back to 600 using the idle screw. I've read several articles on the subject and they have indicated the proper way is to adjust the fuel/air mixture first, and them use the idle screw to adjust the idle. However, this certainly does not mean one should not check the mixture again. I've taken it out for two 10 mile drives and there was no dieseling after cutting the ignition off, so I'm hopping this has fixed the problem. I appreciate all the suggestions/advice I have received on the dieseling problem.