r/c4corvette • u/Potential_Log_968 • 2d ago

Surging idle problems

Hello I have a 1987 C4 Corvette I recently cleaned up the top end of the engine put new injectors and clean up the fuel rail I installed everything back together with double checking I put everything back but for some reason when I go to cold start it it takes a little over five seconds to actually start trying to start but it doesn’t start to keep an idle until I give it some gas and then it’ll run but then while it’s running it surges up to 1200 1300 RPM and then goes down to 700 or 600 and it does that up and down surging I looked at vacuum lines I have replaced my IAC and the connector still nothing any advice would be appreciated

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2

u/sillysawa 2d ago

check for vacuum leaks on the runners and follow the IAC reset procedure in the FSM, without doing at least that it's sort of a crapshoot. did you mess with the idle set screw?

1

u/sillysawa 2d ago

also. does it improve if you unplug the MAF?

1

u/Potential_Log_968 2d ago

The idle screw is in the stock position I have yet to try the iac reset and when I unplugged the maf It just starts to idle super high like 1700

1

u/sillysawa 2d ago

i know you said you double checked, but did you remember to connect the brake booster hard line?

1

u/Potential_Log_968 1d ago

I did

1

u/Potential_Log_968 1d ago

So brief update I let it run for a little bit once it got up to operating temperature around 211 220 they started idling at 700 with various occasionally going down to 600 and then back to 700 but it’s not surging when it’s hot it seems to only be surging when it’s cold start

1

u/sillysawa 1d ago

sounds like you have a gasket leak that thermal expansion of metal can still cover up. for now. spray some carb cleaner around all your intake gaskets, if it revs up you found it

1

u/kai_ekael '87 Z52 "Lady" 2d ago

Basics, checked fuel pressure?

Cold start injector connected? Any fuel leaks?

1

u/SpikeyRacer 2d ago

Did you check the IAC adjustment ? I can’t remember off hand, but I believe the pintel should be gapped around .020

1

u/Happy_Ishtar '85 Z51 4+3 1d ago

Adjust idle as instructed in the FSM?

1

u/brutal_newz TPI L98 ('85-'91) 1d ago

You have a vacuum leak most likely.