Hello,

I am posting here because I am out of ideas.

Before posting, I read a lot of topics on the forum about potential identical issues but with no luck being the same.

My car is an ‘84 Trans Am with the factory LG4 and E4ME engine. The ony mod is it doesn’t have anymore the AIR pump and all the attached lines and hoses. Everything was removed before I bought it back in 2001. I never had particular issues and it ran fine till few weeks.

I rebuilt a factory Q-Jet back in 2021, because I had play in the throttle shaft from the original carb from my car. Put bronze bushings in the throttle shaft. It ran pretty good only when warm, only small issues under choke phase, when cold.

As I wanted to improve that, and try to correct the small mistakes I could have done, I decided to rebuilt another one this winter.
Put again bronze bushings, checked the throttle and secondaries flaps and centered them before reassembly.
Checked the MCS voltage and resistance which were good.
TPS was new and calibrated on the car.

I installed it, but since, the engine is idling terrible. For information, everything was set on bench by the book (fuel float, lean stop, rich stop, choke, idle mixture screws, IAB). I knew I will finalize and adjust the setting on car, with a dwellmter, as the last time.
TPS is set at .48v

Even with the engine at temp (almost 160F on the gauges), the dwell seems to be fixed, most of the time around 20 degrees (I read it on the 6 cylinders scale from the dwellmeter).

When I choke the carb, the reading increases, so it means the meter is working and that the carb / ECM is trying to lean the mixture.
I checked the vacuum lines with a vacuum pump, and unless I did a mistake or forgot something, they held the vacuum without leaks.

Turning the mixture screws CW or CCW doesn’t change a bit the reading. Turning the IAB either.
Advance is ok, with EST connector unplugged, I set it to 0 (even at 6 but it didn’t change anything).
So I am not sure it is in closed loop, even if the engine is at temp.
Trying to jump A & B on the OBD doesn’t light the check engine with engine running, so I can’t see if it’s still in open loop.

Would that mean I could have an issue with the ECM ?
With key ON but ignition OFF, the light is on. I can check the code (12).
With engine started, the light goes off as usual.

I tried to test drive the car, just to help warmed it up a bit, and see if it helps to go in closed loop.
Driving is terrible also. Particularly at low revs and low speed. It’s like the engine accelerate a bit then slow. In comparison, it gave the same impression as driving with oval tires…

Just to gain some time and because I have them already, I will change the coolant temp sensor and O2 sensor, as I saw they can prevent the engine from going into closed loop.

If it doesn’t work, apart from the ECM fault, I don’t know where to look.

Thanks for the advice or tips you could give me, if you have a clue 🙂

Have a nice day !

 

Ok bit of positive news in a way.

I have checked and redo the distributor setting.

Now it idles smootly at 600-700 rpm on D at the gauge when hot. It doesn’t vibrate anymore. It’s back to how it was before.

But when in P, after some times warming and idling, it revvs by itself slowly : 900 rpm, then 1000, 1200 and stops there. All by increments.

It was in a phase during which I was waiting for the engine to be warm and in closed loop, to be able to continue the calibration of the carb : curb idle, check the ignition timing, TPS, IAB with the dwellmeter.

It made me something very surprizing. After have given a tap on the accelerator pedal, with a peak at around 1500 rpm, it kept revving past this value without any action from me, as my foot was not resting anymore on the pedal. I cut quickly the engine around 4000 rpm.

I checked immediately, I don’t see any explanation fot what happened. The linkage on the carb is not sticking, the cables on the throttle shaft are fine too, as is the spring. Nothing prevented the pedal from going back to its normal position (nothing was blocking it or pushing against it).