update: Cold running issues

[monkeyodeath]

Hi everyone, sorry again if this question seems rote, just getting used to M20s...

When it's cold, (Los Angeles cold, mind you, so like 50 degrees or so), the car starts right up, no issues. But for the first 30 seconds or so the idle bounces from 500 to 1000 RPM.

When I give the engine some gas, it stumbles a bit, but after a few seconds trying to rev it up, I can get it to rev smoothly. After I do this, the revs seem to even out and the idle is smoother.

At this point the engine seems to really bog every time I hit the throttle, either to the point of dying or almost dying. Revs drop for a moment then pop right up.

Because of this, I end up killing the car a lot, since getting out of my very sloped driveway requires me to be fast on the throttle. When I do kill it, it's really hard to start up again, and I need to wait for a couple minutes before I'm able to get it started again, and even then it takes a while to get going. Seems to be especially hard when I'm facing uphill.

I'm not throwing any CEL's, so can't speak to that. In previous cars I've had bouncing idle is related to bubbles in the coolant, but I haven't seen that mentioned in the searches I've done, so I'm a little stumped...would an intake leak be the source of all these issues?

[jlevie]

That sounds most like intake leaks. At least that is the place to start by having a smoke test run on the intake.

[monkeyodeath]

An update:

Smoke-checked for leaks and found one at the little cap on the passenger's side of the intake, where the vacuum line runs out to the FPR. Replaced that gasket, and that seemed to cure the starting problems, car starts up flawlessly now.

What's an issue now is that while it runs and seems to hold idle okay, it's running terribly - pretty sure the issue is that it's running pig rich. Smells rich and fuel smoke is pouring from the tailpipe. Runs roughly and dips a bit when throttle is applied quickly. I noticed that even just disconnecting the FPR vacuum line to create an intake leak makes the engine run a little bit better. I wonder if the previous vacuum leak was offsetting how rich the engine was running before.

So far, I've run it through most of the tests you're supposed to do - coolant temp sensor tests fine at the sensor itself, fuel pressure was about 5-7 psi too high so I replaced it with another one that works fine. Pulled out the fuel rail and checked for leaking injectors, no problems there. ICV seems to be okay, cleaned it out. Swapped in a replacement AFM to no avail.

Only thing I haven't checked yet is the O2 sensor...I tried getting the car warm and then disconnecting the sensor, to see if it made a difference...it didn't. Not sure if that's a valid test, but thought I'd try.

So, should I just buy a new O2 sensor? Is there anything else that could cause the car to run so rich? The idle/starting issues have seemed to stabilize, which makes the car more driveable, but I hate to fry the cat any more than necessary...especially since I need to smog test the car this month. I tested it using a multimeter and it failed, but since I can't tap into the O2 sensor while it's plugged into the heater, not sure if that's a good test or not.

[monkeyodeath]

And thanks in advance for helping a noob!

[jlevie]

If the engine is running rough it is misfiring. That will cause the exhaust to be fuel rich even if the engine is otherwise running normally.

The O2 sensor has a finite life and is a scheduled maintenance item that is to be replaced every 100k. If the sensor is at or past the maintenance point, replace it. The rough idle could be a mixture problem, or it could be something else. If the ignition wires, distributor cap & rotor have significant age on them replace them. Then install a fresh set of OE or equal plugs and adjust the valves.

[CarsSuck]

did you try unhooking the battery to reset the ecu? It adapts some, particularly the idle, in the first 5 cycles or so from what i can tell. You've cut off the small intake leak that the ecu has become accustomed to so its gonna be rich until it goes closed loop a few times to compensate.

Am I wrong?

[monkeyodeath]

Didn't even think of that...had the battery on and off as I've been working with the fuel system, but I don't think I've run it enough times to get it to readjust.

I pulled the O2 sensor and it was completely covered in carbon, so I replaced that. The ignition components are of unknown age but definitely not new, and the spark plug wires have W. Germany printed on them, so I'm going to go ahead and just replace that stuff.

I am worried, though, about how long it's been running rich, since I know that's a great way to burn out a cat!