My salvage titled car has had a low idle problem for a long time. It runs great at anything above idle. Here is what I've done:

Checked vacuum hoses for leaks
Checked fuel pressure
Replaced a visibly bad injector
Swapped to a known good distributor
Swapped to a known good ECU
Adjusted valves
Replaced and adjusted throttle switch
Tried 3 different MPSs
Fiddled with the ECU knob
Unplugged the intake air temp sensor
Measured the hot and cold resistance of the CHT
Set timing
Replaced plugs, cap and rotor


What I have not done, but suspect:

Injector seals
Injector flow test


Here is what it does:
It idles fine when cold. Once it warms up the idle adjustment screw has to be all the way out for it to idle at about 800-900 RPM. After a spirited drive, the idle will drop to 4-500, and sometimes it dies. After a minute or so of idling it restabilizes at about 8-900 RPM. (this is with the adustment screw all the way out still) It doesn't sound like it's missing, just idling too slow.

Could bad injector seals cause this problem? I know it's not a main pelenum vacuum leak because that causes a high idle. Would a leak at just one or two injectors imbalance it enough to cause a low idle?

(Yes, I have read Brad Anders' site repeatedly)

OK I have been through this problem myself. When the car is cold it gets it's air obviously from the Aux air control valve. Once that closes up then the system gets it's air from the bypass screw on the throttle body. I had this issue for a time before I decided to pull the unit off and do a real close inspection. I found that the chamber where the air bypass screw goes into was plugged up. I always noticed that when I would screw mine out I didn't hear any air hissing. Once I took out the bypass screw I could see a ton of garbage in there and the gasket was trashed. I cleaned out the area with a pipe cleaner and some good ole carb spray. Reinstalled and my problem went away. Now mine idles perfectly Something for you to think about checking.

actually i had the EXACT problem on my car....adjust timing and dwell....

Oops, I forgot to list a thorough cleaning of the throttle body and air bypass among the things I've done. Timing and dwell are adjusted. I don't think it's a centrifugal advance problem. If it was not advancing enough, it would end up too advanced at idle resulting in a high idle, because timing is set at full advance.

Does anyone know how advanced it SHOULD be at idle? I have an adjustable timing light and can easily check the advance at idle.

i had heard it should ~ the 1.8 timing mark at idle....just here-say though. Pelican tech article states the specific timing thingy

bump?

Trigger points!!! Why does it always have to be trigger points???

I'm not a FI expert, although I do have a great system tester. Another well known 914'er who too professes no expertise in FI, have a running joke that whenever you've got FI problems the first thing to say is "trigger points". Funny thing is the last four times that was said between the two of us, new trigger points and off we go!

I agree with Perry that trigger points are usually the answer. As a matter of fact I replace them first when problems come up. One sign if injector seals are leaking is some backfiring on deacceleration since air get sucked into the cylinder past the seal. You can try to look closely with a light and if you see some sooty black residue around the injector bodies that will tell which ones are leaking. I could never see anything so I just replaced them all. Make sure the plastic tip is still on the injector too as it will cause problems if missing, since the injector will now move around.

The distributor I swapped in had good trigger points and the problem didn't go away. I think it does backfire a tad on decel tho. Hmm.

Your last comment raises the question....How did you know the trigger points are good? I have seen several sets that were tested with a distributor machine and the resistance changes that indicated open/close looked really good, but they were still bad. Another nit picky item, how do you test your vac lines to see if the connections are good? If they are cloth covered then it is best to cut off a little at the end as there can be splits and you can't see them. If they get hard due to heat then they'll look good but won't seal well.

Well the distributor as a unit ran fine on another car. The vacuum lines I cut the ends off of and replaced a few. All the vac lines will either have no affect on the idle or make it go high, not low, even if completely disconnected. (except for maybe the MPS line, which I replaced)

I seriously doubt the trigger points as the idle problem was exactly the same with two different distributors, and the problem only occurs at idle, when the job of the trigger points should be easiest.

How is your FI wiring harness?

I've read that can be the problem with a lot of annoying FI problems.

low idle *could* be the same as a shitty idle....did you check the tps? I have the same problem....if you try to zero that thing out, let me know how you did it....i cant see how you would do that without removing the throttlebody. also could be the wires leading to the trigger points too.

Could be the harness, didn't think of that.

The TPS is not like a normal TPS. I know it's working right because at idle it tells the ECU to use the idle circuitry, and that allows the mixture knob to work. If the throttle switch isn't working the knob will have no effect on idle, but it does. The other function of the throttle switch is to enrichen during acceleration, kind of like a accelerator pump. That's the part that usually fails, and that has no effect on idle.

QUOTE (bondo @ Mar 10 2005, 10:59 PM)

Could be the harness, didn't think of that. The TPS is not like a normal TPS. I know it's working right because at idle it tells the ECU to use the idle circuitry, and that allows the mixture knob to work. If the throttle switch isn't working the knob will have no effect on idle, but it does. The other function of the throttle switch is to enrichen during acceleration, kind of like a accelerator pump. That's the part that usually fails, and that has no effect on idle.

i shoudl install my new one then...