Bad IAC? (Page 1/3)
Stricken SEP 12, 05:49 PM
So I am continuing the tune up my 88 Formula. I replaced the whole distributor assembly, and that seems to have solved the issue of stalling when warm. It is also idling at a normal rpm now (not sure of the number as the tach isn't working). It was stumbling and stalling at idle and when I pushed the clutch in coming to a stop, but from a tip on this forum, I added a ground, and that seems to have helped quite a bit, but it's still not idling smooth and will buck and cough when taking off from a stop. The engine runs nicely when cruising, though. TPS and MAP voltages seem to track correctly and it has consistent ~38psi fuel pressure at idle and increases when I blip the throttle. The EGR diaphragm appears to work, but I'm not sure about the solenoid. I removed the IAC and it had a bit of soot, but not a lot.

Is the IAC the most likely culprit here? Normally, I would just replace any aging parts like this, but since I'm planning on a 3800 swap in the next year, I don't want to spend more on this engine than needed for the near term.
Patrick SEP 12, 07:37 PM

quote
Originally posted by Stricken:

...will buck and cough when taking off from a stop.

Is the IAC the most likely culprit here?



I wouldn't think the Idle Air Control has any bearing on acceleration problems.

How did you check the Throttle Position Sensor?

And when you replaced the distributor, you timed the ignition with the ALDL jumper in place?
Stricken SEP 12, 10:38 PM

quote
Originally posted by Patrick:

I wouldn't think the Idle Air Control has any bearing on acceleration problems.

How did you check the Throttle Position Sensor?

And when you replaced the distributor, you timed the ignition with the ALDL jumper in place?




It's just at initial tip in from a stop. Once off idle a bit, it runs just fine. It also bucks and coughs when in gear at a low rpm. I re-gapped all of the plugs, and that helped a bit, but it still tends to die when I push the clutch in coming to a stop with a/c on. Sometimes it will almost die then recover. It will hunt and search and eventually die just in neutral with a/c on as well.

I inserted some safety wire into the middle wire of the TPS connector and checked that against ground.

Yes, I had the jumper in when I timed it. I tried several times to lock the distributor down dead on 10° for #1 and #4, but it kept shifting to 11° if that makes a difference.
olejoedad SEP 13, 09:29 AM
Some 2.8's like 10°, but I have seen some happier at 11°, 12°, even as high as 14°.......
Are you splitting the difference between #1 and #4?
Stricken SEP 13, 01:25 PM

quote
Originally posted by olejoedad:

Some 2.8's like 10°, but I have seen some happier at 11°, 12°, even as high as 14°.......
Are you splitting the difference between #1 and #4?



#1 and #4 seem to be pretty dead on with each other, maybe .5* difference. I jumped the tach filter, and my tach works. It's now idling around 1600rpm which sounds much higher than it was last night, and it doesn't seem to want to stall.

Patrick SEP 13, 01:32 PM

quote
Originally posted by olejoedad:

Some 2.8's like 10°, but I have seen some happier at 11°, 12°, even as high as 14°




...and just to make it even more complicated, if the outer ring of the harmonic damper has spun on its inner hub, then the timing marks are all wrong. This is not a rare occurrence, especially considering that the dampers can now be 35 years old. It's worthwhile to check that the TDC of #1 cylinder is indeed 0° on the timing scale.

[This message has been edited by Patrick (edited 09-13-2020).]

Blacktree SEP 13, 02:39 PM
Those symptoms sound similar to how my engine behaved when the IAT sensor started going bad. On the V6 engine, the intake air temperature sensor is on the side of the air filter canister. You can check its resistance with an Ohm meter against the ambient temperature, using this chart:



It's OK if the sensor is off by a little bit. But if it's off by a large amount, it can mess with the air/fuel ratio. Mine was off by 60 degrees! The engine would sputter and sometimes die when I came to a stop.

Also, the coolant temperature sensor can be tested the same way. It sticks out the side of the intake manifold, just below the thermostat neck.
Stricken SEP 13, 03:21 PM

quote
Originally posted by Patrick:


...and just to make it even more complicated, if the outer ring of the harmonic damper has spun on its inner hub, then the timing marks are all wrong. This is not a rare occurrence, especially considering that the dampers can now be 35 years old. It's worthwhile to check that the TDC of #1 cylinder is indeed 0° on the timing scale.




That is a very good point that I hadn't thought of.
Stricken SEP 13, 03:27 PM

quote
Originally posted by Blacktree:

Those symptoms sound similar to how my engine behaved when the IAT sensor started going bad. On the V6 engine, the intake air temperature sensor is on the side of the air filter canister. You can check its resistance with an Ohm meter against the ambient temperature, using this chart:



It's OK if the sensor is off by a little bit. But if it's off by a large amount, it can mess with the air/fuel ratio. Mine was off by 60 degrees! The engine would sputter and sometimes die when I came to a stop.

Also, the coolant temperature sensor can be tested the same way. It sticks out the side of the intake manifold, just below the thermostat neck.



I will check this next. I just noticed an interesting phenomenon which may correlate to the IAT being bad. It seems to hold a steady idle around 1500 rpm coasting around 30mph, but when I come to a stop, the idle seems to try to hold 1200rpm, but jumps around alot and occasionally drops down to 500 and recovers up to 1600 again. Rather like it's getting less cool air coming in when stopped.
Blacktree SEP 13, 03:32 PM
IMO, it's best to check after the car's been sitting overnight. Because then, both sensors will be at ambient temperature.