Iac- 2.8l v6

[85_Ciera_Rebuild]

Engine starts fine, but runs on high idle after warmed up until vehicle is stopped. When stopped, I kick it into neutral, then the very fast idle slows down to normal idle.

I ordered a new IAC....it ain't a vacuum leak.

Engine has had a slight missing out at idle, like maybe extra air was being sucked in. Kick up RPM, and engine runs smooth.

Some $15.00 for new IAC shipped.

Never had this happen before, but issue is related to IAC

[Keiths1976]

Make sure it comes with a new o ring for it . I had one car that the o ring cracked and has a hairline split causing it to suck air in and throw off the idle but kicked the engine light on 92 Pontiac grand am 3300 v6

[85_Ciera_Rebuild]

Make sure it comes with a new o ring for it

EGR is not holding a vacuum at idle, so its constantly sucking vacuum (air) at idle.

Engine came with Speed Density software, and apparently, it does not throw an EGR code. In 1988, on W engines (Beretta), GM had a MAF, but it failed, so they used Speed Density, On 1989 W engine, it came with Speed Density, no MAF. (Beretta). I must conclude they fumbled the ball on programming for an EGR code, since I recently plugged the vacuum line going to EGR, and no code has appeared.

I have blocked off EGR's vacuum line, and it seems so far, that the computer is slowly reducing fast idle RPM from 2,800 to now 2,000, each day. Only with a cold start does idle goes high after being warmed up.

I suspect the IAC is within specs, since ECU does respond to a high idle when stopped, by throttling it down to normal idle range. So, I haven't replaced IAC yet, since I think the ECU may correct this high idle in time.

Awhile back, I noticed engine's idle was a bit rough, but off-idle RPM, it runs just fine. Then it progressed to this high idle, after a cold start. Hence, EGR's vacuum solenoid/diaphragm is shot, as based upon my test of plugging its vacuum line.

[85_Ciera_Rebuild]

Tidbit - With old IAC still in place, and without a battery reset, I put engine thru an idle relearn process. Interesting, during cold side of warm-up, it was running a bit higher RPM, then it dropped down to expected RPM as it warmed up. When doing a battery reset, it dies when starting up initially, and then ECU bumps up RPM, after each restart.

I may do a true idle relearn (battery reset) with new IAC, but I wanted to confirm EGR was the issue first.

[85_Ciera_Rebuild]

Conclusion - EGR's diaphragm/solenoid was defective, not the IAC

Finally did a battery reset for Idle Relearn with new IAC installed, with EGR's vacuum plugged. As normal, engine dies initially, which means no vacuum leak, and after restart, ECU kicked up idle. This last summer, I installed new injectors, and saw no issues related to vacuum lines/etc....so, with new gaskets and injector o-rings, there appears to be no air leaks.

Speed Density software did not test for EGR operation...something they missed.

Power wise, doing that Idle Relearn made a difference. I suppose their idle fuel parameters are related to fuel enrichments, until O2 sensor maps them at various RPMs. Before Idle Relearn, open loop was running in fast idle, and closed loop was OK.

[Century7667]

Back in the mid 90's I asked a fleet mechanic about some Lumina's and Cavaliers in his fleet. I hadn't owned my first computer controlled fuel injected car yet. "What get's these cars? The fuel injection or the computers? What do you service on them?" He told me, "Usual stuff, water pumps, EGR, all the physical components. The computers and fuel injection almost never require service." These cars were mostly 3.1 MFPI with some 2.0/2.2 engines in some of the Cavaliers. Time has proved him right in my experience, and those cars in that fleet drove all over Houston pushing 200k on them.

Ken T.

[85_Ciera_Rebuild]

...The computers and fuel injection almost never require service."

Earlier ECUs used with carbs may have had issues...mine shot craps. But, having 1989 vintage ECU, it has not shot craps yet. Bosch had a few years where ECUs would fail as they aged...I don't know if this was related to bad capacitors, like it was in computer world, Capacitor plague, "between 1999 and 2007." In trucking world, earlier ECMs went down with bad caps.

With time, o-rings need to be replaced on injectors, and of course, FPR will shoot craps with time.

Overall in later years, EFIs ECUs/Injectors are trouble free. I'm not aware of injector failures en mass, but there were injector failures.. On semi-truck engines, each cylinder's injector can be tested since earlier days.

Uncle Sam did step in and mandated a minimum detergent package...I do know in earlier days, getting the wrong fuel (cheap fuel with no detergent package) could affect injectors