Help! hard to start when hot

[MARCUS VETTE]

Corvette 1986
Here’s the description of my problem:
· It has difficulty starting or will simply not start at all when the motor is hot
· When I let it cool off for a minimum of 3 minutes it will start if the gas is floored
· I obtain 3 error codes; 21, 34 & 36
· I have changed to following parts: MAF Sensor & MAF cut-off relay
· I have set the RPM at 1500 because it makes it easier to start the car
Marc

 

[Schrade]

Ah yes - the ol' Select Fire Assault Wallet repair...

Hard hot restart could be leaking injectors, or low fuel pressure, or both, among other things.

Get on the codes first. Get the diagnostic procedure printed out from a local mechanic, or get FSM off of ebay. Get static and dynamic fuel pressure tests too. Easy. See youtube for reference vids, post back here for EXACT procedure.

Buying parts blindly NEVER gets repairs done. Never.

 

[MARCUS VETTE]

Thank for your support,
I have already checked the pressure and the leak injector (It’s not that). The only thing I’m sure of is when the motor is hot and I shut it, there’s no pulsation at restart (I checked with a special tool “Light”). If I want to restart, I put a little bit of “break Cleaning” in the intake hose, and the pulsation comes back. The Corvette will start again or I have to wait a few minutes (around 3 or 4 min).
I wonder if this could be caused by a vacuum problem. I suspect it may be due to the line between the fuel tank and the engine. Any thoughts?

 

[TedC]

Do you have a DVOM? Measure the resistance on each injector with the connector off. Post the results. How old are the injectors?

 

[MARCUS VETTE]

Do you have a DVOM? Measure the resistance on each injector with the connector off. Post the results. How old are the injectors?

Results 17 for alls

I don't know:confused

 

[TedC]

Corvette 1986
I have set the RPM at 1500 because it makes it easier to start the car

Please explain how you did this? TPIs do NOT have an idle speed adjustment. If you adjusted the screw on the throttle body, then you adjusted minimum air, which is base line air flow the ECM expects so it can manage the idle speed by moving the idle control valve in/out to programmed idle speeds. There is a set of steps to calibrate the throttle body, and they must be followed in proper order.

 

[MARCUS VETTE]

Please explain how you did this? TPIs do NOT have an idle speed adjustment. If you adjusted the screw on the throttle body, then you adjusted minimum air, which is base line air flow the ECM expects so it can manage the idle speed by moving the idle control valve in/out to programmed idle speeds. There is a set of steps to calibrate the throttle body, and they must be followed in proper order.

I twisted the cap distributor, I don't touch the adjustment. It’s the only way I found to restart My Car. :confused

 

[John Robinson]

Follow the codes.

DTC 21 indicates that the TPS is telling the ECm that the the engine is in a rich condition. Go to this post and read response 8 I think it applies to your situation.

http://www.corvetteactioncenter.com...performance/121778-help-96-lt1-drop-dead.html

Codes 34 and 36 are probable not going to be an issue if you find the tps bad because the tps ground is common for those circuits so they are having sympathy pains with the tps circuit.

 

[MARCUS VETTE]

Hi John

You’re probably very close, I try the fix the TPS at 0,54V. The close position I can have, is 0,62V. If I leave the DVOM, the engine go very well, the go at 800 RMP, the start immediately back. When I put out the DVOM, the idle go at 1600 RPM and they don’t start !!!!

 

[John Robinson]

Check the wires

OK here is the next thing I have found with the TPS. The black ground wire was broken inside the plug. I repaired that and everything was fine for about 1 day then the car went sour again. Again the black ground was broken. Repaired it the second time and again it ran fine for about a day and then it broke. One more fix and then I drove the car from Indianapolis to Milwaukee and back in one day only to have the gas mileage drop off. Stopped at a rest stop and asked a trucker for a piece of wire to again fix the ground wire. However this time I found the blue was broken. Fortunately I was able to bypass it temporarily and got the car home. Upon further investigation I found out that all three of the wires from the wiring harness to the TPS plug were brittle so I Pulled out a connector I had salvaged from a junk yard and transferred the connectors to the TPS plug and now I have everything working again.

I think the reason your engine starts with the DVOM connected is because you are making a ground for the TPS with the meter.

In my experience all three wires to the TPS are very small and probable don't have more than 6 wires inside of them. I think that they get broken when you try to check the voltage. I would bet that most people use a straight pin to perce the wire so they can check the TPS settings. This is probable how they get broken.

My trick again is to use wire and connectors salvaged from a junk yard and Remove the mail and female pins with about 6 inches of wire on them from the connector. Take and strip about 1/2 inch of wire on each piece and twist them together and solder them. Now you have a jumper wire that you can shove into any plug and connect to the other end and you can use the soldered joint to connect your volt meter to the wire.

 

[Schrade]

I twisted the cap distributor, I don't touch the adjustment. It’s the only way I found to restart My Car. :confused

I don't think you can rotate the cap alone. I think you probably rotated the entire distributor, and in the direction of timing retard. Advance timing, and rpm's drop.

Retard timing, and rpm's increase, because the fire is closer to TDC. Advance timing, and the rpm drop is because the detonation is 'further BEFORE' TDC, and hence, more resistance to proceed to TDC in the cycle.

Sometimes in starting a car with extreme advanced timing, the symptoms mimic a dead battery.

 

[vetteoz]

· I obtain error codes; 21

I think the reason your engine starts with the DVOM connected is because you are making a ground for the TPS with the meter.

DTC 21 indicates a problem with the TPS sensor or open circuit on TPS wiring
If the ECM sees a high TPS Volts reading while cranking it will shut off the injector pulse

 

[TedC]

Well, if Marcus floors the throttle body as per post #1, then the TPS is going to send over 4 volts to the ECM, which will stop pulsing the injectors. Timing should be set properly before adjusting the TPS.



Throttle Body Adjustments for 1989 L98 Corvette

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I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).

Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps

 

[MARCUS VETTE]

Hi John and everyone,

I fix my problem with my corvette, finally it was the TPS. I’m very appreciating your support. I’m happy to have a corvette.

Nice summer and nice ride with yours Corvettes

Marcus Vette