Idle Troubleshooting

[gbmidyear66]

Well, the new injectors cured my rough idle - it now is super smooth (need to try the "nickel test", and my "seat of the pants" dyno indicates a measurable performance improvement. One thing that isn't better is my idle speed.

If I am coasting in neutral, Idle will be about 1200, once I come to a stop - it will drop to 900 (as indicated on tach). I have the Haibeck Prom and flowmasters. I believe the 1200 RPM behaviour is "normal" on a 90 - and doesn't particularily annoy me.

I expected my at rest idle to come down with the new injectors, but it hasn't. I mean - 900 is not a huge deal - but clearly something is not 100% right. I have read through a number of posts and believe the culprit could be ... TPS sensor, dirty throttle body, and of course the usual suspect - vacuum leak.

My question is .... (while waiting for my LT5 Service supplement to arrive) - what troubleshooting steps/sequence should I take to isolate the cause (ideally simplest test to hardest?)

Thanks

[GOLDCYLON]

Clean the IAC valve First.. cost is nothing Its on the passenger side of the air horn. Check your MAP sensor hoses back side of the plenum and the one under the ECM tray on the drivers side. GC

[rhipsher]

I had the same problem when I put my whole top end back together after porting and powder coating. When I would pull up to a stop my idle was between 1500 and 2000rpms. But if I gave the gas peddle a quick tap with my foot it would drop down to normal 900 so I just shot some white litheum gease on the throttle body plate rod and problem solved.

[gbmidyear66]

Thanks for the good ideas guys. Will follow through on them

When I removed the plenum I noticed the MAP sensor hose was unplugged at one end. I glued the hose to the rubber boot (end that connects the plenum) - I had thought that may have been why my idle was high previous - obviously not.

[rhipsher]

Yeah these cars don't like it when the map sensor is unplugged. Mine runs like it's choking on its own vomit if its unplugged.

[gbmidyear66]

OK, where to go next?

So far no luck fixing my idle. Have done the following...

- checked for vac pump operation with key on, engine off. Runs briefly, shuts off for extended period, no vac leak on secondaries
- removed IAC, it was really carboned up, cleaned with carb cleaner, replaced - no effect
- I have the connection to the PVC's tie wrapped down tight
- The fuel press regulator vac connection is good - as are lines at top of plenum (either side) I took them off individually and blocked them - no effect.
- checked that secondaries are fully closed, there is no effect if I try pushing them closed further when engine is running
- placed vinyl binder over air horn - that slowed it down all right (had to yank it off before engine ate it)

- should I not have vacuum on the line to my secondary MAF sensor connection (under the ECM)? I remove this connection and their is no suction on the vac line with engine running?

[tomtom72]

Hi, in answer to the last Q: No, no vacuum to differential MAP till the secondary vacuum control solenoid opens upon ECM command. That hose actually comes off of a t-ee at the left actuator's feed line.


On the old rag over the carb trick.....on an LT5 we need to use one of those old fashioned cardboard loose leaf binders, the rigid ones, and even at that be careful as an LT5 will eat that also. I made a piece of 1/4" plywood with some rubber on one side and a handle for that test.

Before I got the samco PCV kit I used to do the same thing with mine. I can't say if it was a cure for the possible small leaks that whole deal could offer as the rubber ages? I assume that you checked the double connector at the T/B left side where the PCV suction starts from? Those leak badly sometimes. Mine does a bit, I'm gonna eliminate it the next time I have the plenum off.

Not to be a smart a$$. I assume you cleaned the IAC passage in the T/Body also? Oh, use throttle body cleaner and not carb cleaner around the throttle body when you have to clean. The old carb cleaner will attack the DAG behind the throttle blades.

When you are reading the idle rpms, are you using a scan tool or just the dashboard? I've read where some of the 90 tachs are not that accurate down at idle speed. Just a fwiw. Mine kind of agrees with my scan tool, sometimes.

Only other place I can think to look is the left side of the plenum where the water exit for the T/B de-icing and the PCV hard line cross each other. It's an interference fit, and some of us have more interference than others. Mine does and I suspect it's a coolant leak into the cylinder #1 injector holes on my injector housing. That's another thing I'm gonna eliminate. I suspect that the plenum will not sit tight on the I/H from that interference of the two pipes.

Other than that stuff, I'm fresh outta thoughts on your idle issue. Oh, by chance did you use the carb cleaner to clean up the throttle body bore? If yes, check to see that the DAG is still intact.


Tom

[QB93Z]

Can you describe exactly what idle issue you are trouble shooting? Are you trying to get the 900 rpm idle to be lower or are you trying to correct the idle "hangup" when coming to a stop sign?

Some other thoughts:

I worked on a car where the idle was wrong because a guy who did the powder coating had not put the throttle mechanism back together properly.

I manually adjusted the idle speed using a small allen wrench on the throttle plate return position stop but I don't I don't recommend that unless you believe that the throttles were taken apart and put back together wrong.

Try turning the air conditioning on and off at idle and see if the ECM is adjusting the idle speed as it should.

The idle speed can be adjusted by a tuner by changing the ECM chip parameters.

Jim

[tomtom72]

Forgot to mention this.

On our 90's there was a rewrite of our EPROM's by GM to address customer complaints, among which was a hot idle and coast down idle issue. The new PROM, AYBK, replaces the OE PROM, AUAH. This was a TSP, not a recall, to be done if the customer complained. The TSP is over at our NetRegistry site in the maintenance section under the recall & TSB tab if you want to read the laundry list of sins from the AUAH program.

I saw in your sig that you have a PROM. I made the assumption that the basic BIN file is a copy of the AYBK file as that is the preferred file to write to by most Chip makers. I know Marc Haibeck uses that file as his base for his chips.


Tom

[gbmidyear66]

Tom, thanks for clarifying the secondary MAF sensor - the behavour I see now makes complete makes sense. The IAC passage looked clear.

I'm going by the tach, I may need to break down a get a scan tool, any recommendations on that subject would be welcome.

I will investigate the front left plenum connections closer - the original dual connector has been replaced by individual hoses there - so that is definitely a suspect. I think I will also loosen and re-torque to spec the plenum - just to be sure and remove my "repaired" MAF connection and check that it is not the issue

Jim - engine is original paint, not ported, I believe Mark Haibeck was last person to have had the plenum off and service the engine so I do not believe it is likely that issues are related to something being monkeyed with. I was reluctant to play with manual idle speed adjustment as otrhers have also indicated this should not need to be touched. I am going to probe the TPS and just check that voltage is correct.

I am trying to get the idle down to where it should be (650 range)

I have the Haibeck PROM, it was installed in about 2003, I doubt Marc would have sent the car home without correct Idle speed - so I don't suspect that.

Thanks for your suggestions guys

Glenn