Mechanics

ENGINE RUNS POORLY WHEN WARM, STALLS AT STOP LIGHTS

Mitsubishi Other

I have a 1991 Mitsubishi Eclipse, standard transmission, 1.8L engine. 123,000 miles. When the engine is warm and RPMs go below 3000 the car sputters and tries to die. Usually in stop and go traffic, or when approaching a stop light. It is intermittent, so it doesn't always do it. I have had this problem for over 1 year. I have replaced the following things: Fuel pump, fuel filter, fuel pressure regulator, fuel injectors, plugs, wires, distributor cap, rotor. I have removed and thoroughly cleaned the throttle body, pcv valve is OK. Approx 1 year ago I replaced the alternator, O2 sensor. Had a re-manufactured engine installed (long block). There are probably more repairs I've done but I just cant remember them at the moment. I'm pretty much at my wit's end with this one. Please help.
AD
Avatar
Prweaver
October 5, 2007.




Hello!
Did you check/replace/clean the IAC?
(idle air control)?

The IAC helps regulate the amount of air that enters the engine at idle.

And controls the rpm`s at idle

How about the TPS (throttle position sensor)?

Is the check engine light on?

Tiny
Fisherman
Oct 5, 2007.
Thanks fisherman, for your response.
I visually inspected both the TPS and the IAC. I did a resistance check on the TPS, it was within specs. I removed the IAC from the throttle body (still connected to wiring harness) and turned on the ignition and verified that it extended, but I don't know of a way to test it other than that. Is there a way to give it a more thorough test?
The check engine light works, but it never comes on while driving.

Tiny
Prweaver
Oct 5, 2007.
Hello!

Well for now just clean the IAC meanwhile I`ll lock for the correct procedure to test and calibrate the IAC on your car (every make and model are diferent)

but. I been thinking. Have you check the EGR?

Here you will find how the EGR works on your engine: In a typical automotive spark-ignited (SI) engine, 5 to 15 percent of the exhaust gas is routed back to the intake as EGR (thus comprising 5 to 15 percent of the mixture entering the cylinders). The maximum quantity is limited by the requirement of the mixture to sustain a contiguous flame front during the combustion event; excessive EGR in an SI engine can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and NOx emissions. A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms: Reduced throttling losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
Reduced heat rejection. Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
Reduced chemical dissociation. The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is relatively minor compared to the first two.
It also decreases the efficiency of gasoline engines via a few more mechanisms: Reduced intake charge density. EGR tends to heat the intake charge. This means a bigger piston or stroke must be used to induct the same amount of fuel and air mixture. This results in a bigger and heavier engine.
Reduced specific heat ratio. A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston.
EGR is typically not employed at high loads because it would reduce peak power output, and it is not employed at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle.

That way if its not operating correctly (stuck open) will lead to a rough idle and lack of " power" lets check that!

And let me know!
AD

Tiny
Fisherman
Oct 6, 2007.
Actually, this engine doesn't have an EGR system. Or if it does, I have never been able to find it. There is only 1 vacuum nipple on the throttle body, and it goes to the Evaporative Emission Control system. I replaced the Canister about a year ago (one of the repairs I forgot about). I think I tested the purge solenoid but I don't remember.

I was thinking about the IAC. As I understand it, the IAC motor extends and retracts a piston which mechanically opens or closes the throttle valve, correct? Anyway I experience stalling not just at idle, but also when pressing the accelerator gently, or maintaining a constant pressure on the accelerator. If the RPMs are below 3000, it starts to die, then catches itself momentarily, then stalls again.

Tiny
Prweaver
Oct 6, 2007.
I just rigged up a home-made diagnostic scan tool with some wire and an LED. It sent back 2 codes repeatedly. According to my Chiltons manual they were: Throttle position sensor, and Motor position sensor. Under the TPS it says to check 1) Harness and connector, 2) TPS, 3)Idle position switch.

I have checked the TPS following their instructions, but I find no instructions on testing the Idle position switch. I'm not sure how to test the harness & connector other than look for cracks or broken wires.

Is the motor position sensor the same as the camshaft position sensor or the crankshaft position sensor?

Tiny
Prweaver
Oct 6, 2007.
OK, here's an update. I adjusted the position of the TPS, and cleared the codes by disconnecting the negative battery terminal. Now it drives just like it used to (dies when warm) and there are no error codes.

Tiny
Prweaver
Oct 6, 2007.

AD