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 :hello:

[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.

:cheers:
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]

:o 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.

:cheers:
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

[tomtom72]

Glenn,

Thanks for not thinking I was trying to be a wise guy with the IAC passage.

TPS = 0.54 v at idle, hot. It's adjustable, but I don't know what the range of adjustment is, sorry. I never had to adjust mine, but the hold down screws are in slots and it looks like there is a useful bit of adjusting that can be done.

I bought a used MT-2500 off e-bay. :o I'd kill for a Tech1A. The real issue is finding all the software for all the different systems. Old technology & all for an antique car.
It doesn't exactly agree with my tach when I scan the car at hot idle. My tach down there appears to read low to me anyway. Also, my idle seems to fluctuate between 650 and 675 when I scan it, sometimes even as high a 700 and as low as 600. The TPS reports in at 0.54 v.

:cheers:
Tom

[gbmidyear66]

I'm down to about 800 RPM now,maybe I should just declare victory and move on?

The tube that connects to the dual PCV valve connector was a little loose - tie wrapped that.

Re-torqued all the plenum bolts

Re-tie wrapped the short rubber tube at the front left plenum, I could change idle speed just by wiggling it.

Re-tie wrapped MAF sensor hose.

I think my next step is to pick up a scanner and a small bottleof propane, this is getting a little frustrating.

[gbmidyear66]

I am still fighting with my high idle, and am currently out of ideas.

I bought an Innova scanner off of EBAY which I am waiting to arrive so I can check the IAC value, TPS voltage, the target idle of the ECM, and to confirm the tach RPM.

Meanwhile - I tried the binder over the air intake, it killed the engine. This leads me to believe that my efforts with tie wraps, etc to eliminate vac leaks was successful.

I manually opened the secondary throttle plates - there appear to be nice black rings (presumably DAG) in behind

My TPS sensor is installed at the max clockwise end of the adjustment range, any adjustment counter clockwise only increases my idle speed.

I alkso check for vacuum at the dipstick (which I read somewhere should exist), but did not detect vacuum there - is this a clue?

Any fresh ideas or insights much appreciated.

[tomtom72]

Glenn,

Not very sure about the "vacuum" from the stick's tube? Crankcase should be somewhat pressurized via power stroke blow-by, no?:icon_scra

If you use some straight pins, you could probe the TPS connector & use a DVOM to read it's voltage. Use the straight pins to go in thru the back of the engine harness connector, this way we make no holes in the wire insulation. It's a bit tedious, but it will work. I did it once using regular phone wire. The other alternative would be to make up a T-Y harness to go in between the TPS and the engine harness with an extra female connector to use as a probe point.

I'm very loath to even suggest this "option". We have a idle stop screw on the underside of the linkage. The factory adjusted it and it's to be left alone for the duration. If someone played with that before your ownership, that could be a contributing factor to this high idle issue. Most people don't even know about that screw so I would reserve that till the bitter end. It's not easy to get to with the T/Body mounted on a complete, running engine anyway. I think there was one of those special GM tools to mess with it on the car.

You covered all the usual suspects, and very well at that! I'm thinking along these lines: high idle rpms = too much idle air(?) from bad IAC(?) or bad TPS(?), but if the ECM thinks the coolant is cold, it commands IAC to open(?), then the ECM's temp sensor maybe at fault? But that only makes sense if the motor is cold, or in open loop.(?)

Closed loop high idle = too much air(?) = vacuum leak ( we don't have any!), therefore, suspect could be fueling(?). Why is fueling rich? = bad O2's(?), bad IAT sensor(?) I'm not sure if the fueling was out that a couple hundred rpms could be smelt if too rich or too lean in either open or closed loop operation.

I'm going to go look in the FSM in section 6E to see if something jumps out at me; be back later on coffee break!

:cheers:
Tom

[tomtom72]

Okay, break time...in section 6E under intermittents ( pg. 6E3-B-7 ) on "incorrect idle speed" they refer to TPS voltage and linear operation of the rotating element. IAC system check @ chart C-2C, ECM grounds, and alternator output & grounds, throttle linkage is not binding, PCV valves are not clogged.

As an aside on the PCV's. I have found thru my 5 yrs of ownership that this motor is very aware of how clean the PCV's are kept. By virtue of our inherent excessive oil vapor generation I have taken to replacing my PCV's every winter. I find that they become sticky after only 5k miles in my car. It's not that they don't work, but the sound is no longer that nice clean metallic click when you shake them. I've no idea if this is relevant here or not.

Okay, I looked up the IAC diagnostic chart. Do me a favor, are the PCV's okay for sure? Yea, the chart is a bit complicated and requires some special electronic gizzmos to perform. There are work arounds for those of us that do not have a Tech 1A and all the attachments....I just have to figure them out...sorry I'm not that quick with this electronic stuff:o

Let me know what's up with the PCV's.
:cheers:
Tom

[A26B]

My TPS sensor is installed at the max clockwise end of the adjustment range, any adjustment counter clockwise only increases my idle speed.

I alkso check for vacuum at the dipstick (which I read somewhere should exist), but did not detect vacuum there - is this a clue?

There is something amiss with the TPS. Virtually all I have ever seen, are roughly in the center of the adjustment slots, i.e. the sensor is situated level. Your TPS may be sick.

Re: vacuum @ dipstick seal. My initial thoughts are this won't be the case with the LT5. There are 2 fresh air/filtered vents, 1 on each cam cover, for crankcase ventilation. At idle vacuum, it doesn't seem likely/feasible that there is or should be negative pressure in the crankcase that would be detectable.

[gbmidyear66]

Tom

Thanks for all your efforts on my problem, much appreciated. I'm going to try to get to NAPA before close tonight and pick a couple PCV valves up - I expect mine are original, and given their cost, I should just replace anyhow.

Yeah, I was aware of the idle stop adjustment, but just didnt want to go there until after having gone over and eliminated any faults evident with the scanner.

Jerry

Thanks for clarifying the absence of vacuum on the dipstick


I'm going to replace the PCV's and run the scanner (which I'll get early next week - have to cross the border and pickup at the parcel depot) and report back. It seems that 90% of these idle issues are vac leaks, but it looks more and more that my case is one of the other 10%.:cry:

Glenn

[tomtom72]

Glenn, you're welcome.:thumbsup:

I have thought about what Jerry wrote about the TPS...and damn if mine isn't level and so are all of the ones I've seen. Hummmmm....maybe there is something to that?

Have a good weekend!

:cheers: