Hello I have a 1970 Chevy C10 that I have put the motor and computer from a 1989 TBI into (5.7 truck motor).
At idle I have a problem with the idle fluctuating and I can't get it to go away.
The motor is new (rebuilt), new gas tank and fuel pump with pressure gauge and I'm running 12-13PSI.
There are no vacuum leaks, just put a new intake manifold gasket (a mechanic installed this for me and check the for vacuum leaks with a smoke detector unit).
I've rebuilt the TBI, new gaskets, and injectors.
New distributor, ignition coil, plugs and wires.
New "Painless" wiring harness to rule out bad grounds.
New Computer and all sensors are new (IAC, TPS, CTS, O2, EGR) except for the MAP sensor.

I should note the fluctuation began when I rebuilt the TBI and replaced the AIC and TPS at the same time. I saved the old AIC and put it back and that didn't fix anything, but I accidentally threw away the old TPS and trash pickup was the next day so I couldn't put the old TPS back on to see if that was the culprit.

Note: I have aftermarket headers and exhaust with glasspacks. The O2 bung is on the exhaust right at the header/exhaust joint as opposed to being right on the end of the header where the stock location is (I have on occasion got a lean O2 code but very rarely and it goes away quickly when it happened).

I just ordered a SnapOn Brick to maybe see if certain sensors aren't working (waiting for it to arrive).

I think maybe there is an issue with the TPS but won't know until the Brick arrives.

I'm at my wits end off getting this fluctuation to go away, so any help would be greatly appreciated.

 

Did you rebuild the entire TBI assembly or just the injector body? Did you remove the throttle plates?

You don’t need a scanner to test the TPS - monitor the signal circuit at idle with a voltmeter. Anything outside 0.4 to 0.6 volts will cause the ECM to think the idle speed needs adjustment.

When you get your scanner, check and see what the IAC counts are, first at key on, engine off and then at idle. If they’re fluctuating, either the control circuits are having contact problems or an input used for IAC count control is throwing off the ECM.

 

I guess just the injector body, replaced gaskets, fuel pressure regulator, and injectors. I didn't mess with the throttle plates.
For the TPS, which wire of the 3 should read that?
Will do! Thank you for all of this information! Much appreciated...

 

The original wire color was dark green, but if the pigtail has been replaced, it should be the middle wire.

 

Awesome, thank you. I'm taking it in to the mechanic tomorrow to get new front coil springs installed, but I'll check all of these things when I get it back.

 

I also don't have a VSS (vehicle speed sensor) hooked up to the ECM. Would that cause any problems...

 

It shouldn’t cause any problems with idle speed control. The ECM for a 1989 only utilized VSS input to control the TCC, cruise control, or shift indicator light for manual transmissions.

 

Got the scanner and TPS is at 0.54
For the IAC counts: 145 key on engine off, Engine running fluctuates around 80 (79, 80, 81)
Scanner also fluctuates between lean and rich exhaust (not enough to make the check engine light go off but...)
What does this part of your message mean? “either the control circuits are having contact problems or an input used for IAC count control is throwing off the ECM.”

 

I would consider those IAC counts normal. Minor changes to the counts are expected - if it had been 10 counts or more, there’s reason to suspect circuit or control issues, but for them to remain in that range means the ECM is trying to maintain the idle based on the information it’s receiving.

The exhaust switching back and forth from lean to rich is normal. It has to do with how exhaust gases exit the combustion chamber.

​​​​​​Regarding the control circuit contacts and ECM inputs - I’ll post details later as I don’t have time at the moment, but electrical connections and accurate sensor input are vital to the ECM controlling the engine, not just at idle but through all speeds and loads.

 

I have a brand new wiring harness and computer so I wouldn't think there were contact problems... I don't know...
I did find a vacuum leak and fix it and that made it way better but it's still not perfect. On my scanner the truck never goes into closed loop, is that normal?