Cold Start Performance Issue – Need help

[Ray]

Thanks in advance for the help.

I have a 1994 Corvette with aprox. 40k original miles. Stock. I’m experiencing cold start sputtering and lag at any accelerator pressure until warmed up to normal operating temperature, then it goes away and runs fine.

Items just replaced: (issue occuring the same before and after relaced items)

New Optispark distributor
New waterpump
New coolant temperature sensor
New plugs
New plug wires
New air filter
New pullies and belts

Mechanic said it’s possibly running abnormally lean when cold and as it warms up the computer compensates hense why it runs fine when warm.

Any ideas what could be causing this issue on cold start?

Topic

[Ray]

… yeah thats what I’m thinking. Not ready to spray flammables on my engine.

How do I do a smoke test?

[Austin]

1. NEVER EVER clean your MAF with something like a q-tip, they’re so sensitive it takes VERY LITTLE to destroy one.

2. Stop worrying about flammables, go to the store buy a can of carb cleaner and start spraying down vacuum lines, intake, plenum, etc. Your engine will need to be cold when you do this. It does not take much spraying, We do this daily at my shop and never worry about igniting. Now listening for idle change is an okay way to do it. The best way to do it, is while your spraying watching live data and see if a certain spot you spray makes the upstream o2 sensors run rich.

3. A typical smoke machine cost a couple thousand dollars. You can make one out of a Halloween smoke machine. Take off your brake booster line at the brake booster, or any other vacuum line you have access too and pump smoke into it WITH THE ENGINE OFF AND COLD. Replace where ever the smoke comes out.

keep us posted.

[Ray]

Okay so after fixing the issue on my other car it’s back to the 1994 Corvette. I have not had a change to check for vacuum leaks yet.

In addition to the sputtering and lag during cold start, I also had the following experience..

The other day while on the freeway for about an hour (outside temperature about 80d) the car started to heat up. Around 195d is coolant operating temperature for the car. As it crept higher well past 200d the car began to rev up and down from 1000rpm to 3000rpm by itself and lurching forward. I pulled over and shut the car off. I did not see coolant leaks. This happened once before when I had been driving for about an hour. It appears to only overheat under load when it’s been running for a while (hour+).

My mechanic couldn’t figure out why it would overheating, as he has done all the work on the car in my original post. The car only has 40K original miles but the fact that it is 20 years old we ordered a new radiator thinking it may be blocked coolant flow.

Could the lag and sputtering during acceleration and overheating be caused by the same or different issue? If different, what do you think could cause the overheating?

[Ray]

I was reading up on symptoms of head, exhaust, intake gasket leaks. Could that cause both lagging performance and overheating? My understanding is there is no way to tell unless it’s taken apart and inspected. I haven’t noticed any unnecessary smoke after it’s been running.

[EricTheCarGuy]

Those may be symptoms of air in the cooling system. Since you just had a water pump replaced, it might not be a bad idea to check for that.

As to your original issue, I’m wondering if perhaps you might have an issue with the idle air control valve. If that was not working correctly it could cause cold starting issues as well as the changes in idle at the stop light. I’d still check for air in the cooling system first as that can also effect the coolant temp sensor readings.

Keep us posted.

[Author]

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