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Dr. Hamman
Dr. Hamman, Technician
Category: Classic Car
Satisfied Customers: 6286
Experience:  I have 30 years experience repairing, restoring, and customizing cars, and pickup trucks.
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Chevrolet C4: I've got an 85 Corvette that bogs at wot l.

Customer Question

Hi I've got an 85 Corvette that bogs at wot l. Runs good on part throttle. Have a solus scanner tps is .54 at closed throttle. Fuel press is adjustable at 42 psi. Step down on it & it won't go over what part throttle rpms are. Any suggestions ?
Submitted: 1 year ago.
Category: Classic Car
Expert:  Mike S. replied 1 year ago.

Are you saying the voltage is .54? If so, what is the voltage at wot?

Customer: replied 1 year ago.

Tps is fine. .54 closed 4.48 wot

Expert:  Mike S. replied 1 year ago.

Why didn't you mention that?

Anything else you didn't mention?

Expert:  Mike S. replied 1 year ago.

Did someone leave the Electronic Spark Timing wire disconnected?

Are there any codes?

Is this the tuned port? If so, does this have the map sensor or air flow sensor?

If you know if the engine vin is an 8, P, J or 5?

Customer: replied 1 year ago.

Vin 8 maf is new no codes. I'm a 37 years' veteran on GM electronics specialize in Cadillac . Fuel press is good 42 at idle. Never drops below 38 at wot. Timing is right on. What am I overlooking?

Expert:  Mike S. replied 1 year ago.
Don't know. I will opt out.
Customer: replied 1 year ago.

Need aC4 tech to help diagnose an 85 . Do you have anybody ?

Expert:  Dr. Hamman replied 1 year ago.

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.



Have you watched the fuel pressure as the engine cuts out, sometimes fuel delivery problems can create a symp***** *****ke this. Also have you looked to see if the spark fades as the engine cuts out.

Customer: replied 1 year ago.

Engine does not miss fuel pressure does not drop under 38 psi and holds steady. Timing advances to 45° and starts to retard slightly. The knock sensor is not installed. This is a 383 stroker that runs Sonoco fuel. Runs great at part throttle.

Expert:  Dr. Hamman replied 1 year ago.

Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.


I would be concerned about whether the absence of the knock sensor is effecting the ESC (electronic spark control) unit. The whole engine control setup can drop out of balance with modifications. I would check the power and ground connections on the engine computer, power starvation can do very strange things. I would perform voltage drop tests on the power and grounds on the computer. Look at the spark as the engine cuts out. Since the timing is verified, if the ignition is involved you should be able to see the transition as the engine cuts out. As the RPM's rise the ignition is under more stress to provide adequate ignition. That said the injection might be involved. On the injectors there are two banks fired as a group. Both banks have their own power source, both banks are triggered independently on separate circuits of the engine computer. From the description, you have fuel delivery to the injectors. I would question the sparks strength, and the computers output to the injectors. If you have something an electrical meter to count pulses, hook it up to one injector bank and then another. Watch to see if the injector pulse diminishes as it cuts out, or if the pulse broadens. If the pulse broadens it points toward spark problems. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.


I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.

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