Hello all! I just purchased a 1990 C4. I got it from an estate sale super cheap due to a rough idle and stalling issue. It's been sitting about 1.5 years. It has fresh gas though. Engine was rebuilt 60km's ago. Mods included, 10.5 pistons, LT1 cam, hv oil pump, double roller chain, ported heads. Has no knocks or clicks, good compression, good head gaskets, good fuel pressure. Engine starts up well but runs rough, stumbles, seems to get worse when warming up. Like a vacuum leak but no hissing sounds. Possible EGR valve? What happens if I disconnect the vacuum line and plug it? Possible TPS or IAC issue? Anyone else have this issue who could help? It's a very nice car that had a lot of money put into it...just runs like a pail of s***!
1990 corvette runs rough
- Thread starter Darryl L
- Start date
Update! replaced IAC module and cleaned out throttle body. Car runs perfect! Roasts the tires too! I can't find any other mechanical or electrical flaws after a hour driving! Looks like I may have fluked off and got a good car! Thanx to some guys lack of interest and a bitchy wife who wanted her parking spot back, I got it for $3200cdn. The things even nice to look at lol!
He guys I need help again. I replaced the IAC module and did the reset. It runs beautiful till it get hot then it starts flooding stalling (black smoke) and the fans don't come on so it starts to overheat! I turn it off and wait 10 min it starts right up and runs smooth till it gets hot then does it all over! The fans worked before I switched the IAC O
Over. Anyone have any idea what has changed after installing and setting the IAC? I did remove the TPS plug to get access to the IAC plug. I did the reset procedure, power on unplug and power off replug thing to both. Any help would be appreciated ttyl
Over. Anyone have any idea what has changed after installing and setting the IAC? I did remove the TPS plug to get access to the IAC plug. I did the reset procedure, power on unplug and power off replug thing to both. Any help would be appreciated ttyl
Hi Darryl. Welcome to CCF. Sounds like you got a pretty great deal on your Vette.
There are lots of C4 guys and good Mechs on here to help you out. Unfortunately I’m not one of them.
The white C4 looks good and I expect soon you will have it sorted out. Then a few days of playing and under the cover for the winter.
Derek
There are lots of C4 guys and good Mechs on here to help you out. Unfortunately I’m not one of them.
The white C4 looks good and I expect soon you will have it sorted out. Then a few days of playing and under the cover for the winter.
Derek
Unfortunately, I am not update on trying to help with your problem, hopefully someone here can help you out. You did pick up the vette for a very good price anyway.
Thanx! I'm looking forwaforward to the challenge! My first c4 vette! I had a few c3's and a c5.Hi Darryl. Welcome to CCF. Sounds like you got a pretty great deal on your Vette.
There are lots of C4 guys and good Mechs on here to help you out. Unfortunately I’m not one of them.
The white C4 looks good and I expect soon you will have it sorted out. Then a few days of playing and under the cover for the winter.
Derek
That was a wicked deal that will make a lot of us on here jealous. I wish I could be of more help but all I can sugget is trying to reseat the TPS plug (although I seriously don't think that is the problem) and if that hasn't helped, disconnect the battery and reset the computers. This is grasping at straws but right now off the top of my head I can't really think of anything else.
Thanx for the info. I must tugged something or broke a wire while installing the IAC. I'm gonna plug away at it today with some advice. Sure runs good when its not acting up. Good power, runs smooth till it gets too hot lol! Internals seem excellent no knocks or clicks, lots of compression and power.
Update! After a day of chasing wires I have the fans working correctly (old corroded contact at relays beside rad) a buddy and I also spent the day going over what is done previously to see if I missed anything. So far it's not a heat issue, not IAC, not TPS, not EGR, not temp sensor, not a MAP sensor, no vacuum leaks, good fuel pressure, correct voltage on all connectors as per test! Possibly ECM? Anyone else have a similar issue that could offer advice? Help!!
I was all excited to see you fixed the car.......until I read your next post. 
So, to sum up, it runs great when cold, but once warm, it gets too hot, even though the fans are now working or does it just run rich? If overheating, does it stay cool for a while then over heat or just overheat off the hop? Could it be the thermostat not opening? Coolant level (I know, this would be the first thing to check but I have to ask
)? Is the water pump working well?
As for running rich, once warmed up and in open loop, the O2 sensors may be pooched and sending signals for too much fuel. If they're ok, check the fuel injectors themselves.
So, to sum up, it runs great when cold, but once warm, it gets too hot, even though the fans are now working or does it just run rich? If overheating, does it stay cool for a while then over heat or just overheat off the hop? Could it be the thermostat not opening? Coolant level (I know, this would be the first thing to check but I have to ask
)? Is the water pump working well?As for running rich, once warmed up and in open loop, the O2 sensors may be pooched and sending signals for too much fuel. If they're ok, check the fuel injectors themselves.
Hi! After getting the fans to cycle correctly, it is not a heat related issue. The car runs at normal temp now and the fans working well. I installed a new IAC because of a rough idle. The rough idle issue is fixed as well! Now it starts excellent and runs like new and can even be driven short distances. Lots of power, no smoke, good throttle response, smooth idle at lights. Then all of a sudden, out of the blue, it will stall at low speeds (while slowing down for a light etc) and when restarted will run like it's flooded with black smoke from exhaust (ultra rich). I leave it off a minute or two and it fired up and runs beautiful for another while. This has happened at all points in the start warm operating tep cycle. It seems totally un related to a heat issue or warm up cycle as the car runs normal temp now. In the hunt for a solution, I've checked the EGR, TPS, MAP, IAC, ECM temp sensor, injectors volt spec, fuel pressure, Re calabrated IAC and TPS as per spec, fuel presure regulator, ran all voltage specs, checked for vacuum leaks, broke wires etc... all the basic easy stuff. I think I need to recheck fuel pressure with a newer gauge to be safe as the gauge I used was old and I'm not sure how accurate. I'm down to a evap canister, possibly a O2 sensor (no O2 code though), ECM, possibly ignition module, or?? Kind of a rough problem to solve lol. I will re do many of these tests again just to be safe and not miss anything. I'm not a great mechanic but can patch up a thing or two on occasion lol! The thing runs so good when it's working correctly! I forgot to mention, it's not just a driving issue it will screw up just idling in the driveway. Sometimes it will idle for a long time before it launches into it flood and die issueI was all excited to see you fixed the car.......until I read your next post.
So, to sum up, it runs great when cold, but once warm, it gets too hot, even though the fans are now working or does it just run rich? If overheating, does it stay cool for a while then over heat or just overheat off the hop? Could it be the thermostat not opening? Coolant level (I know, this would be the first thing to check but I have to ask
As for running rich, once warmed up and in open loop, the O2 sensors may be pooched and sending signals for too much fuel. If they're ok, check the fuel injectors themselves.
My buddy had the same symptoms recently with a ‘90 he was plannng to flip.
In his case it turned out to be the Throttle Position Sensor(TPS).
He got the car running so well he decided to keep it for a while..
New rims, tires, paint stripe, polish, and a good detail...
He now has a C3, C4 & C5 in the garage.
I hope you find your issue soon.
Graham
In his case it turned out to be the Throttle Position Sensor(TPS).
He got the car running so well he decided to keep it for a while..
New rims, tires, paint stripe, polish, and a good detail...
He now has a C3, C4 & C5 in the garage.
I hope you find your issue soon.
Graham
Hi all! HELP! I replaced the IAC and TPS modules. It still runs well then floods itself to death. Switch it off for a few secs and restart and it runs fine until it floods again. Last time it ran for 30 min before flooding out.its not a heat issue it' not a TPS or IAC issue. Anyone know of a good corvette mechanic in saskatoon?
Hi Darryl. I see a few possibilities that may be a cause of your problem, In these years, GM had weak coolant sensors. they often get short or open at certain temperature, most of the time, when they do, the pcm will, by default, see the temperature of the engine at -40celcius so it will spray a huge amount of fuel thinking its super cold. This sensor have 2 wires, a yellow and a black. Disconnect the sensor(engine off)and at 20Celcius you should read about 3400 ohms, -7Celcius shoul be around 13500 ohms, hot engine at 100Celcius it should be around 185 ohms and if it get bad it will show you like 100000 ohms (reading for -40 celcius) or completly open circuit. The coolant might be bellow trottle body. The other possibility I see might be a oxigen sensor that get bad when hot but the coolant would be the first thing I would check.
I picked these info for a engine code ''8'' L98. If you can post your serial number or at least the eight and thent digit. Is the car automatic or manual. There are many different wiring diagram specific tho each engine and transmission.
If you have acces to a OBD1 scanner, you could read the temperature of that sensor true the pcm, if you have a bad reading with the scan check the sensor in case the pcm or wirring is defective The coolant sensor does not give the info to your dash so you probably have a good reading of temperature as the dash might take is reading on another sensor. Also if the pcm see -40Celcius it may not start the fans, who need the cooling fan when its -40C
Good luck and post your results. I am in my big season so I may not be there every day but I will try to help the best I can.
Stephane
I picked these info for a engine code ''8'' L98. If you can post your serial number or at least the eight and thent digit. Is the car automatic or manual. There are many different wiring diagram specific tho each engine and transmission.
If you have acces to a OBD1 scanner, you could read the temperature of that sensor true the pcm, if you have a bad reading with the scan check the sensor in case the pcm or wirring is defective The coolant sensor does not give the info to your dash so you probably have a good reading of temperature as the dash might take is reading on another sensor. Also if the pcm see -40Celcius it may not start the fans, who need the cooling fan when its -40C
Good luck and post your results. I am in my big season so I may not be there every day but I will try to help the best I can.
Stephane
Hi! Thank you for the tip. I will run the test tomorrow. Mine is a 1990 L98 automatic. I'l let you know how it does.. It's winter here, so it may be hard to regulate ambient tempeature as the door has to be open to run it. It' sounds like a good possibility. It runs so nice when it's working right. No internal noise at all. Has good power to lol. Thanx again!
Darryl...
Hi Darryl. This sensor work as your choke use to work on your old carburated engine so ambient temperature should not be a probleme as long as the ohm reading of the sensor fallow the coolant temperature. warm it up until the problem is present, turn the engine off then take the reading on the 2 pins of the sensor. Also check for traces of coolant in the connector, these sensor use to leak. I remember back then I was using a variable resistor to simulate the coolant sensor, this way I could see if the PCM was working OK with a good reading.
Stephane
Stephane
Darryi I have owned a 1990 ZR-1 for some time but never had your problem. I think one of the best people to ask this question to would be Marc Haibeck www.ZR1specialist.com. He knows these cars inside and out. Good Luck
