Regarding high idle (measured 964 in park, 760 in gear).

Here are my observations:

- Idle is very smooth, no rapid oscillations. Occasionally it will change slowly up then back down, but not like a surge. It's a slow change and smooth.
- Acceleration is excellent. Better than before. MPG seems noticeably higher than before. No hesitation or other indication of a lean condition.
- Idle stabilization routine runs to completion but changes nothing.
- Vacuum is 20. If I pull of the brake booster hose I can feel great suction. If I take my thumb off the end, creating a major vacuum leak, the idle does not change. Put thumb back on the hose end - no change. What does that indicate?

So, what are the candidates?
- The spare MAF. Lord, it makes the car run well (brisk rev, no hesitation, great power all the way up to redline). Oh, well. My plan is to get a rebuilt MAF from Louie and send him my original MAF which looks brand new but seems to have failed. Then I'll swap that in for this MAF, and if that fixes it, great.
- Idle stabilizer. The idle stabilizer makes a rich clack when tested. Still, a stuck stabilizer proved to be Stan's finding with a high idle recently.

Any other ideas or troubleshooting suggestions?

 

Bill,

This sure sounds like an air leak, too much idle air. It would be useful to know what the ISV signal is, if you can get your hands on George's Sharktuner. If it is a big air leak then ISV% would be low as LH tries to reduce idle speed.

Have you checked the throttle cables? Is the throttle plate returning completely to its mechanical stop? If the cables are messed up then the throttle might be held open a little.

And of course the classic split ISV hose under the intake...

Cheers, Jim

 

Jim:

I'm puzzled by the good vacuum and the lack of effect of creating a sizable leak. Perhaps this vacuum is not that good for 900+ RPM.

Throttle cables are proper - a little slack, and the idle switch trips. Also, I can see the idle switch trip on the MPG readout (90.00) when I let off the throttle above 1400 RPM.

If the rebuilt MAF doesn't resolve it, I'll probably put in a wideband and have a look with the ST.

Odd - two failures at once: MAF and vacuum hose or ISV.

It sure runs well, excepting the high idle.

 

Bill I mentioned this before but the plastic connectors that plug into the sides of the MAF boot can crack and crumble,
if they are old I would suggest to dive in and replace them.

Yes they are sorta bitch to replace ,
NOTE I actually reenforced the new parts with brass tube procured from the hobby store,
I supergluded them into place and it was a slip fit

 

Or just the Sharktuner, without the wideband. With a good MAF the LH will be bring able to bring the mixture to stoich (14.7), so the wideband won't tell you much. What would be interesting is what is happening with the ISV signal.

I think you are correct about two problems, but maybe not both at once... The LH tries to compensate both fuel and idle. Maybe the MAF has been going south for a while, but the LH was able to compensate the fueling as long as things stayed within its range. The idle would still be OK, even with poor mixture, as long as the ISV stayed within its range... and an air leak would have helped keep the idle up, and masked the poor fueling. But fix the fueling and suddenly the problem is reversed, and the ISV runs out of range in the other direction.

I don't see how a MAF by itself could cause a high idle. Low idle yes, by screwing up the mixture to the point that the engine barely runs, and there isn't enough ISV range to fix it. But unless there is another source of air, the ISV should be able to control the idle speed with a proper mixture.

The problem with closed-loop systems, like the LH's idle and fuel adjustment, is that they can happily cover up problems with no external symptoms, until the adjustment runs out of range. Then things fall apart. That makes diagnosis a real bitch, what seems like a sudden change is just a slow failure that finally hits the stops and became visible. The nice thing about Sharktuner is that it shows what is going on inside the LH. Even without the wbo2 you can tell if the LH is "happy" with the fueling an idle.

Well then drive it! Who cares about a high idle?

 

Cool! That is a great idea, thanks for sharing!

 

LH brain uses the ISV opening and closing to set the idle, but isn't there a screw still on the throttle body?

If said screw is adjusted too high idle even with ISV closed, you get high idle and who knows what else. Same goes for air leaks with the added bonus of letting in unmetered air.

Regards bad MAF detection the feeling I am getting is that LH doesn't do it, but maybe could with a software change. When Bill sends his in for rebuilding, first it gets tested? Might be interesting to see how it failed.

 

Dan: The 85-86 throttle has an idle plate bypass screw. All of them have a throttle stop, but that has never been messed with. Leaving that alone.

Stan: I got a good look at the plastic hose fittings on the MAF boot. They are OK. The hoses and fittings under the intake are not original. I was in there every 2 years when I had the supercharger.

Jim: Yeah, I was thinking of doing the Sharktuner like you say w/o the wideband to get a look at the ISV. I have everything to do the intake, so I might just pull it for a look. Thanks for the insight on how the LH system works. Louis will check my MAF, even if that is not the primary cause. In the meantime the car is very drivable asi is. The high idle seems to be doing no harm. A bit more clunk going into gear initially, but not bad and shifting is as always.

 

Bill when you get the intake off remove the ISV and use a pick to move the vane see if it moves freely or if it has a bind,
the one I cut open looked brand new on the outside,
but I think some engine washing got water into the lines thus rusting this part otherwise its a magnetic assembly so no parts to wear out like the armatures on the 85/ 86 ISV valves

 

Will do, Stan. I haven't ever washed the motor.

It idles OK in limp home mode leads me to hope a freshly rebuilt MAF may take care of this even though it shouldn't.

 

Can you pinch one of the ISV hoses closed?

 

Well, Dan, I didn't try that, but I did spray brake cleaner around and under the intake manifold. No response except for a little near the throttle plate shaft, which is not a surprise and not significant. Also, being that creating a large vacuum leak does not raise the idle, I guess the ISV is a good suspect.

You'd have to squeeze 11 or 15. They are rather stiff even if you could reach them with a forceps. Good thought, however.

 

I'm just thinking if the ISV is all the way closed and idle is high, its an alternate source of air.

 

I'm following you, but there is no "alternate source of air" into the intake. I tested the intake for leaks pretty well. So, now my bet is on the ISV itself, even though I can hear what sure sounds like the door opening and closing when I run the test. If I could close off hose 11 and 15, and the idle dropped, that would nail it as the ISV door stuck open. I can see a bit of 11 and 15 from the front, but my recollection is they are pretty heavy hoses. Maybe I can get a forceps on one of them enough to partially shut it and see an idle drop. Thanks for the idea, Dan. The only other possible cause is the MAF, but the experts don't see how it could do that. The car sure runs terrific.

 

My 88 is having the same high idle issue. Pretty much the same numbers you have. I just finished an intake refresh that included a new ISV, so I suspect it is not that, although sometimes that last thing you touched..... I think my next step is to blow some smoke into the intake manifold and look for air leaks, although I usually think of air leaks as making the idle drift and in this case the idle is very solid.