TPI REVES AS IT STARTS

[grumpyvette]

My 91 coupe has been revving to roughly 2800 rpm when starting the engine (cold or hot) and settles down to 800 quickly.

I suspected a vacuum leak but couldn't find one and both shops that I have taken the car to couldn't find the reason for it.

No error codes, just a temporary high rev.

Anyone experienced this before or have a suggestion on where to begin the diagnosis?

Took the car back to the shop today...

Smoke test on vacuum hoses, all good.

Replaced Idle Air Control Module (definitely needed) but still didn't fix the revving issue.

They suggested replacing the MAP sensor. Put new one on, still didn't fix issue (so they put the old one back on).

Removed throttle body, no hang ups there.

New diagnosis is to replace the ECM.

This particular shop cannot do that repair so the search begins for who can and what will it cost

The ECM is cheap enough at 115 to 150, but lord only knows the hours of labor it will take to replace it.

Got the new (re-manufactured) ECM installed today. The car fired right up with the high revving.

Did NOT reinstall the hypertech chip that the previous owner had put in. Looked up the number on the chip and it's not supposed to be sold in California anyway.

Honestly didn't notice any difference with the chip out.

Drove around for 40 miles to get the new computer to "learn" the settings and everything seems to be back to normal.

I'll have to drive around some more before I go for smog on Monday but it looks like this swap solved the issue.

The mechanic was great, took him about 25 minutes to take the old one off, pull out the prom, remove the chip, install the prom in the new computer and put everything back on the car.

Changing the computer sounded a lot scarier than it turned out to be.

I find bits of this thread rather amusing, keep in mind the basics,
your engine rpms and any increase or decrease in those Rpm levels
are the result of increased,
OR DECREASES IN AVAILABLE AIR FLOW RATES...THINK ABOUT THIS!
NO CHANGE IN FUEL DELIVERY < FUEL PRESSURE OXYGEN SENSOR OR MAF SENSOR DATA OR IGNITION TIMING WILL CAUSE THE ENGINE TO GAIN OR LOSE MORE THAN A COUPLE HUNDRED RPM AT most AND GENERALLY WAY LESS.
if your engines jumping up to 2800 rpm its because theres been a momentary, or intermittent increase in air flow that provides the air flow thats allows it, the fact that it drops back down indicates the reduction in air flow, now remember this it TOTAL AIR FLOW entering the intake plenum, the increase and decrease are not necessarily from the same source but its likely they are or are at least related.
assuming your not dealing with a vacuum leak..
what controls the air flow entering the plenum?
why its the throttle body and the IAC located under it, so its those two components OR THE CONTROLS AND SENSORS THAT MAKE THOSE CONTROLS FUNCTION THAT SHOULD BE WHERE YOUR LOOKING

viewtopic.php?f=32&t=168

viewtopic.php?f=80&t=728&p=43477&hilit=camaro+sensors#p43477


viewtopic.php?f=32&t=1401

 

[87vette81big]

Coolant temp sensor works perfect on my 87 vert Grumpy.

Its been checked by me.

I am done fooling with EFI.

Real power wanted. 800-1000HP.
Hellcat.
Not a single proven build to compete with it Chevy without going bankrupt.

Why I am back on The Pontiac 455 Bandwagon.
9'S hit 20 years ago with factory iron heads, blocks & cranks.
Normally aspirated. No nitrous. No power adders