Choppy idle - cold start hunting, Vac. leak??

[docaudio]

I have an '87 Targa w/ 107K on the clock. 4 years ago Andial did a top end rebuild, and other than that, nothing else major has been done. For the last year or so, I've noticed the following:
1 - Choppy idle. It's like have some pretty stout cams (which I don't). I'm thinking vacuum leak, but where are the trouble spots?
2 - Part throttle surging. I'm thinking air flow meter needs work. I have a replacement headed my way- used - that I'm going to use to compare readings between the two to see if the potentiometer wiper arm needs work. Should I also be looking at the O2 sensor?
3 - Cold start idle hunting. I don't know if this is typical, but at cold start, it will fire up fine, then slow to below normal RPM (500-600), then you'll hear this noise like a valve opening, and the idle jumps up and then settles a few seconds later and is steady, although choppy. It's done it forever, so I'm not sure if it's anything abnormal.
4 - Hot start problem. If the car sits in the sun on a fairly warm day (+80 degrees), it is very hard to start if recently run. Typical scenario - going to lunch. When I go to start it after lunch, it cranks for 10-15 seconds or more and then sort of 'fades in'. It really seems like fuel pressure is slowing building. This is the only time is does anything like this. Fuel pump? Fuel regulator?
I know this is a giant laundry list, but I'm thinking several of these things are related. My first area of concern is vacuum leaks. What is a safe and effective way to track those down? The other area is the air flow meter and it's potentiometer.
Thanks for any ideas!
-Bill

[ischmitz]

The idle issues could be due to a number of reasons

- misadjusted idle mixture (bypass screw on the AFM)
- misadjusted idle stop (on the throttle body)
- failure to actuate the idle microswitch or damaged switch

The AFM can be bench-tested. There is a nice website from FR Wilke explaining the details. You need to hook up a 9V block battery to the common and the power supply pins and then measure the voltage on the output pin while actuating the flap. If there are no dropouts the AFM is fine.

Vacuum leaks sure can be an issue. What comes to mind are the seals from the intake runners to the heads, rubber lines, etc. But if you had a top-end only 4 years ago I'd hope that the seals are still fine. Still, check with starter fluid to see if there is a leak somewhere

The hot-start issue could be a bad check valve. The hot engine causes the fuel in the injector supply lines to boil if the check valve can not hold fuel pressure. You need a pressure gauge to check how the fuel pressure holds when you shut down the engine.

Ingo

[Flat Six]

My $.02: 1 and 3 sound like stuck/intermittent ICV; aged/brittle hoses around ICV can also cause vacuum leaks w/symptoms as you describe. Many other potential sources of vacuum leaks, of course, but this is where I'd start. FWIW, I think you're right to check the O2 sensor; if it's cycling between open loop/closed loop operation the part-throttle surging can be a symptom. The hot start #4 is a little puzzling; if happens all the time (hot or cold) I'd say it's possibly a residual fuel pressure issue (Bentley 240-9). But if just hot start/restart yo might want to check your cylinder head temp sensor. Also, the next time this hot start condition happens, unscrew your gas cap to see if it 'whooshes' -- indicating it's not venting properly. A longshot, but improper sealing / ineffective venting can create all kinds of wierdness.

Hope this helps; good luck.

Dale

[docaudio]

Thanks guys for quick responses! I'll follow up on the bench testing of AFM and the other tests.
This may be a long shot, but is there a way to monitor the Motronic ECU through a cable and laptop - maybe with a little data logging? Does it throw codes like an OBDII system? I've got a laptop in the El Camino for the Holley EFI system and it sure is nice to see everything.

[amschnellsten]

I have the exact same problem with my 84. I changed the head temp sensor, no vacuum leaks, new ref sensors but still there. I have a check valve but I have not put it in yet. I am hoping that is it.

[ischmitz]

The 3.2 doesn't support data logging like later models.

The most common issue is the idle switch not working due to poorly adjusted throttle linkage. Test that first. Next on the list is idle adjustment (mixture & throttle position) if no vacuum leaks are found.

Ingo