I've been hunting down this problem since I bought the poor neglected C4 a few weeks ago. I've searched every thread I could find for help identifying the problem and performed Many suggested tests, checks and repairs. Nothing seemed to help. Then I Finally found the answer that identified Worn Out Injectors! I hope this helps anyone banging their head against the wall diagnosing this problem.

I happened to come across posts on South Main Auto Channel that described exactly the same behavior, symptoms, diagnosing, troubleshooting and testing that I've been doing the last few weeks...to a T. The 'Vette was even the same year as my '89! He actually diagnosed and nailed down the problem! Then in the next 1hr20min post he proceeds to perform the detailed defective Fuel Injector replacement. Great detailed information and easy to follow for those new to Fuel Injector R&R.

The first one is 'Chevy Corvette - No Start When Hot (The Diagnosis)' (

You Tube

) Where he begins by interpreting and confirming the owners descriptions of the problems and poor performance. He does this by test driving and reproducing the symptoms that the customer reported. Then with basic diagnosis checks of the Big Three systems (Air, Fuel or Spark) to determine which one is the cause. In this case the initial tests indicated that the fuel system was Not the problem. However subsequent tests and odd symptoms lead to worn out injectors. Miles and Heat caused Ohm readings to drop into single digits for at least half of the injectors to the point of shorting out. Then as they cooled the ohm reading would rise and they would pulse again. ie. The car would start and run okay for a while, then start running crappy as it reached operating temp. As he said, "These injectors are done...".

The second one is Chevy Corvette - No Start When Hot (The Fix); (

You Tube

)
If you don't plan on replacing your injectors yourself, you can skip this one. I an old mechanic and enjoy doing my own wrenching. He performs the entire repair while narrating every step with easy to understand explanation of what he is doing and why. Great Stuff! Finally what I've been looking for!

 

Eric O is "The Man" !!!...one needs the right diagnostic equipment to troubleshoot this properly !!!

 

One of the first things we usually suggest is checking the Coolant Temperature Sensor (CTS) when a Corvette has Hot starting issues. When the CTS fails it can send an incorrect temperature that us used in calculating the air-fuel ratio.

If the sensor fails thinking it is below 0* the car will add extra fuel to enrich-en the A/F ratio to act like a choke on a carburetor.

When the sensor fails indicating a higher than actual temperature it will reduce the amount of fuel and run very lean.

The CTS is located on the front bottom area of the Intake manifold and has three wires attached to it. It used a 5 volt "reference voltage" and then a signal wire and lastly a ground wire. Be sure it is connected and reading correctly. Inside the Factory Service manual is a procedure for checking the CTS. You measure the resistance and compare it to the chart. For a specific resistance value will show you the temperature it is seeing. If you need it I would be happy to download a copy of it.
Replacing the Injectors is a fairly straight forward job. I have installed a set in my C4 and they made the car idle so much nicer. having the right Fuel Pressure is the most important thing of all in the fuel system.

If your Oxygen sensor is over 24 months old be sure to change it, they wear out and make Corvettes do strange things. They are supposed to be replaced every 24 months or 50 miles. Old Oxygen sensors need to be replaced regularly. The new oxygen sensor will make the Corvette run better and more efficiently.

Best regards,
Chris

 

First thing I'd look at for a poor warm/hot start is fuel injectors. Usually one is leaking all its fuel into the cylinder at shutdown, basically flooding it. First thing to do is hook a pressure gauge up at the rail and watch the FP after shutdown.

 

this.

ohm. and fuel pressure noting leakdown. and then cts.

 

FirstVettFun, if you want a hand or a buddy to mess with cars, I have a group fo C4 guys who live in Southlake and Grapevine. Hit me up if I can point you in the right direction.

 

Agreed, fuel pressure and leak-down were some of the first tests I did.
The FP relay prime on start -> 44psi cold, then settles at solid 38psi at idle to operating temp. Leak down test held for around 45 minutes before dropping 20psi from 44psi relay ignition prime (so still holding around 25psi after 45 minutes). Additional leak-down tests blocking feed line and return line to test Pump, Check valve and Regulator. So fuel system appears to hold good pressure with no injector leaks or line bleed back.

Next I performed cold injector testing...
Ohms: All injectors reading right at 16 Ohms cold.
Pulse testing: All injectors pulsed on Continuous Pulse so I proceeded with fuel pressure drop pulse testing with 44psi starting prime pressure.

Cyl #, 1 Pulse 400 MS, 10 MS Pulses timed burst, 3 1/2 MS Pulses timed burst
---------------------------------------------------------------------------------------------------------------
1, 28psi, 28psi, 40psi then 34psi ???
2, 27psi, 29psi, 34psi
3, 28psi, 30psi, X - pulsed but no pressure drop
4, 27psi, 34-29-29psi, 34psi
5, 28psi, 30psi, 35psi
6, 27psi, 29psi, X - pulsed but no pressure drop
7, 28psi, 30psi, 33psi
8, 27psi, 30psi, 40psi ???

As you can see the single 400 millisecond pulse was close but not within 1/2 psi of each other. Then things get ugly during the faster timed 10 MS burst with #4 inconsistent results. The 3 1/2 MS timed burst was a mess with half the injectors totally bugging out.

Now remember, these are 'Cold' injector tests... the state at which it starts and runs okay for a while until it warms up. Then runs crappy and won't restart. I planned to repeat the injector tests in the 'Hot No Start' condition but I ran out of daylight and it got too damn cold. I'll probably get that done tomorrow. I'll be interested to see how many injectors thermal short with a big Ohms drop. And since the Cold pulse pressure test shows some variance; the Hot tests results should be interesting.

 

Thank you for this suggestion! Yes please download a copy of the chart for me, I'm working without a FSM at the moment. I had no idea that the CTS could cause this... or that it existed. I'll plan to test it after I complete the injector testing in the 'Hot No Start' condition. Then I guess it's the Oxygen sensor.

This C4 has been so neglected I really doubt the O2 sensor(s?) have ever been replaced. Or any thing else for that matter. Heck, the last 'mechanic' that changed the air filter wrote the date and mileage on it with a sharpie. Isn't that fantastic?! I've never seen that level of conscientious detail. Unfortunately the date was 10-27-14... i've only had it a few weeks.

 

That sounds like fun. A group of C4 victims er. Owners eh? You know what they say,... 'Misery loves company'. Yup I've been bitten by the 'Vette bug and by gosh I'm going to get this thing running and have some fun with it! Coincidentally, my neighbor has an '86 383cid C4 project. He gets his motor back in a couple of months. Maybe we can both meetup with you guys sometime?

 

No it should not be the O2 sensor, the oxygen sensor is not used for startup. You should only suspect a failing O2 when your fuel mileage is starting to go off. Otherwise the O2 is basically good until forever. It will rarely ever set a code until its really bad, you should notice a drop in MPG well before then. Basically if youre still getting 22-25mpg on the highway, its fine.

There are timers in the ECM for cold/warm/hot startup based on coolant temperature. These timers must expire before the computer will look at anything the O2 puts out and drop into a closed loop.

Startup is Open loop, basically all CTS and preprogrammed timing tables in the computer. So its a mechanical fuel delivery problem or the CTS, it wont be the O2.

 

Okay, i'll move on to testing the CTS next.
Any thoughts on the Cold injector pulse/pressure test readings? I'm thinking that the individual Ohm readings and pulse/pressure tests (Cold & Hot) asses the mechanical function of the injectors that the CTS has no control over. While the CTS inputs to the ECM affect the control signal to the injectors, it can't affect worn out, failing or shorting injectors.
And of course it's not out of the question that I've got both problems. It's certainly running badly enough to make that assumption?

 

Well I pulled the CTS and tested it. First the connector w/ignition On showed good reference @ 5Vdc. Brought the sensor in the house and ohm tested w/ ice, room temp, and boiling water. Ohm variance around 5500 ohms, 3500 ohms, 470 ohms resp. I don't know the exact temperatures but the switch was definitely responding across a good range.

So I'm beginning to run out of ideas guys. I still suspect worn out injectors and I still need to redo my injector test in the 'Hot No Start' condition. But I want to get it good and Hot by driving it. Unfortunately I have to do that at night since it's not insured or registered yet. So I'll finish that up tonight and compare Cold and Hot (No Start) results.

Btw - I also replaced the ICM when I did the tune-up. I know from experience that ignition modules can die slowly and cause intermittent ignition problems as well as Hot No Start condition until it cools down. Then they magically work again...for a while.

Any other ideas?

 

Okay I finished the injector tests in Hot No Start state. The resistance actually looks great as all tested @ 17 Ohms. The pulse/pressure drop test is a different story however. The pump produced 1psi higher starting point when hot @ 45psi. Single 400 millisec pulse produced a wide variance and #6 wouldn't pulse, then the 10 millisec test pressure drops are all over the board! On the final 3.5 millisec test all failed to pulse hot with No Pressure Drop.

Cyl #, 1 Pulse 400 MS, 10 MS Pulses timed burst, 3 1/2 MS Pulses timed burst
---------------------------------------------------------------------------------------------------------------
1, 29psi, 30psi, X - fail w/no pressure drop
2, 26psi, 25psi, X - fail w/no pressure drop
3, 29psi, 34psi, X - fail w/no pressure drop
4, 26psi, 24psi, X - fail w/no pressure drop
5, 28psi, 28psi, X - fail w/no pressure drop
6, X - fail w/no pressure drop, 29psi, X - fail w/no pressure drop
7, 28psi, 26psi, X - fail w/no pressure drop
8, 28psi, 28psi, X - fail w/no pressure drop

So I'm guessing that these injectors are Done! Interesting that the resistance is spot on within 16-18 ohm range Cold or Hot. I expected much different ohm results, but the pressure drops are way outside being within 1/2 psi of each other. They're pretty wacked...