1985 corvette starts and shuts off after 3 seconds?
IAC?
TPS?
vacuum leak?
IAC?
TPS?
vacuum leak?
85 corvette starts & stops
- Thread starter Nelson84
- Start date
Pull troubles codes, report back with what you find.
When you first turn on your ignition, the fuel pump starts for two seconds, pressurizing the fuel rails. Once the engine shows oil pressure, the fuel pump restarts. Once the engine stops running, check the schrader valve on the passenger side fuel rail for fuel pressure. If there is no pressure, the oil pressure switch may not be registering pressure and telling the fuel pump to run. Check the oil pressure switch. I believe it's the one beside the distributor.
- Thread starter
- #4
It will stay running if I hold the gas pedal down a tiny bit. So I ordered a IAC sensor and will check the tps to see if it’s at .54
the part is on delivery from amazon. Will check back after I exhaust all leads.
the part is on delivery from amazon. Will check back after I exhaust all leads.
whats the history of the car. any recent work done. does it have fuel pressure when it dies?
the oil pressure switch is moot, the drp’s from the hei that the ecu sees result in energizing the fpr. they are a backup for one another.
If it runs when you give it a little throttle, that tells me the logical place to is either your throttle blades are too closed or the Idle air bypass valve has a problem.
TB blades is easy, just a little tweak to the stop. But its not normally a thing that cones out of adjustment without owner input”.
the Bypass valve can be a couple things:
1. failure. Its a little stepper motor with a pintle and they do die after a period of time. Replacement is the only cure in this case. It usually only sets a trouble code if its a “hard” fault. Meaning the stepper motor has a hard electrical fault. The ECM doesn’t know where the pintle is normally, so an out of adjustment condition usually won’t set a DTC.
2. Carbon/soot blockage. The passages can become clogged with engine soot. A good cleaning is all thats required. No DTC for this fault.
3.out of adjustment. The IAC has a spec’d length the pintle must be. This is another thing that usually doesn’t come out of adjustment without outside influence. No DTC, but it might set off others that are related to the IAC failure.
In theory, the throttle blades can be hard closed and the engine will run because the IAC should be able to bypass enough air to keep the engine running. But any of those things above can negatively impact that.
Of note is the IAC only comes into play at idle. When you’re anywhere off idle, the IAC is no longer a player because you are holding the TB open.
If thats all good, them move on to checking other systems.
cheers
TB blades is easy, just a little tweak to the stop. But its not normally a thing that cones out of adjustment without owner input”.
the Bypass valve can be a couple things:
1. failure. Its a little stepper motor with a pintle and they do die after a period of time. Replacement is the only cure in this case. It usually only sets a trouble code if its a “hard” fault. Meaning the stepper motor has a hard electrical fault. The ECM doesn’t know where the pintle is normally, so an out of adjustment condition usually won’t set a DTC.
2. Carbon/soot blockage. The passages can become clogged with engine soot. A good cleaning is all thats required. No DTC for this fault.
3.out of adjustment. The IAC has a spec’d length the pintle must be. This is another thing that usually doesn’t come out of adjustment without outside influence. No DTC, but it might set off others that are related to the IAC failure.
In theory, the throttle blades can be hard closed and the engine will run because the IAC should be able to bypass enough air to keep the engine running. But any of those things above can negatively impact that.
Of note is the IAC only comes into play at idle. When you’re anywhere off idle, the IAC is no longer a player because you are holding the TB open.
If thats all good, them move on to checking other systems.
cheers
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I disagree with what tour is suggesting.
do not touch the throttle blades on an L98 unless its part of the comprehensive
“minimum idle set procedure”
there are systematic ways to solve runability issues on c4’s, where in the old days with carbs we may adjust the resting position of a butterfly,....not so on the computer controlled cars.
example, if you open the tb butterfly (they shipped with a plug from the factory. so if your torx bit adjustment screw is exposed, its been messed with) alone, in isolation from a complete minimum idle procedure, then the computer with command the iac to close as compensation. Then, when the user commands a/c to come on, the car has a mich larger probability to die or almost die due to the fact that the computer had to close the iac so far because the butterflies were opened further than they are supposed to be.
it goes deeper....if your car is full of small vac leaks—most un refurbed c4’s are, especially L98’s) , then the iac will ALREADY be closing itself off to compensate and bring the stoich back into spec. further exacerbating the effects of the solo adjustment to the butterflies.
and another further step into the rabbit hole..and this one is a pita (and not possible on the 85 for any fure 85 readers who find this post) grab a scanner (not a code reader) to ensure that during the min idle set procedure, your iac count is 25-30.
i discourage you you from willy nilly “tweaking” anything. thats likely how your car ended up with the issue in the first place.
im happy to assist you if you like. im in calgary.
btw, it sounds like maybe you already did the MIS and damaged a wire in your tps. So u likely already know the above information!!
Cheers and goodluck!
do not touch the throttle blades on an L98 unless its part of the comprehensive
“minimum idle set procedure”
there are systematic ways to solve runability issues on c4’s, where in the old days with carbs we may adjust the resting position of a butterfly,....not so on the computer controlled cars.
example, if you open the tb butterfly (they shipped with a plug from the factory. so if your torx bit adjustment screw is exposed, its been messed with) alone, in isolation from a complete minimum idle procedure, then the computer with command the iac to close as compensation. Then, when the user commands a/c to come on, the car has a mich larger probability to die or almost die due to the fact that the computer had to close the iac so far because the butterflies were opened further than they are supposed to be.
it goes deeper....if your car is full of small vac leaks—most un refurbed c4’s are, especially L98’s) , then the iac will ALREADY be closing itself off to compensate and bring the stoich back into spec. further exacerbating the effects of the solo adjustment to the butterflies.
and another further step into the rabbit hole..and this one is a pita (and not possible on the 85 for any fure 85 readers who find this post) grab a scanner (not a code reader) to ensure that during the min idle set procedure, your iac count is 25-30.
i discourage you you from willy nilly “tweaking” anything. thats likely how your car ended up with the issue in the first place.
im happy to assist you if you like. im in calgary.
btw, it sounds like maybe you already did the MIS and damaged a wire in your tps. So u likely already know the above information!!
Cheers and goodluck!
here is the minimum idle set procedure. its wordy. just go through it methodically and u will be good.
on the 85, there was a tsb to set the idle higher from the originally low 450 up to 550.
www.digitalcorvettes.com
on the 85, there was a tsb to set the idle higher from the originally low 450 up to 550.
How to Set Minimum Idle on your C4
How to Adjust your Early C4 TPS and Idle Speed -- 1 of 1 Date Published: 2001-10-01 Submitter's Name: Lars Grimsrud Email Address: [email protected] How to Adjust your Early C4 TPS and Idle Speed by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair &...
www.digitalcorvettes.com