The fluctuating idle is back! (Beating the dead horse -- Longish msg)

[CrossT]

After a half year of relatively steady idling, my 86 3.2 DME coupe idle has started back to its old habit plus some!

About six months ago I replaced the CHT and O2 sensors, plugs, wires, dist cap and rotor and the problem went away.

Now the problem is back and more pervasive as well as erratic. Before, it would fluctuate from 800 - 1100 or so but only after a medium temp restart (i.e., after sitting a brief while). Now, it fluctuates when cold, warm, or hot. Strangely, it now will start to fluctuate after a minute or so as I am sitting at a red light, but very erratically (not rythmically). It's more than annoyance now as it will stall.

I tried to 'equate' the idle as previously suggested on multiple posts on this subject, but it even fluctuates with the B & C contacts bridged (it was rock steady with these bridged before). As The idle levels approach the same, the car fluctuates regardless of whether the ISV is turned on or off.

I've checked for a audible click when the throttle is taken off idle, and there is. So, I assume the ISV is asked to operate at idle.

The ISV is less than 50K old and the DME is a rebuilt unit of the same vintage.

I am at a loss here as to where to go look [except to a repair shop, of course]. It would be good to be able to swap/try another known good DME and ISV, but that's a lot to ask of anyone.

The car just passed emissions with a CO reading that was near the limit. All other reading were well below.

Runs real well at other than idle . . .

Anyway, suggestions would be very welcome.


todd
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86 cpe

[scottb]

I have the identical problem in my 84, even though I've replaced the same components you have. It's been suggested to me that the mixture is off. I intend to have my wrench check it next time the car's in the shop. Have you tried adjusting the mixture?

[Wil Ferch]

Why not try some simple things first..like running a few bottles of Techron through it ( added to gas tank)? A hint of a sticking ISV is hitting it with a rubber mallet to see if it affects the idle...

Also, check around the intake manifold area for unmetered air intake leaks. I've heard you can use MAPP gas squirted judiciously in that area to see if revs rise. Bec careful however with any fuel type gas sprayed in the engine area..have an extinguisher handy. Better safe....
---Wil Ferch

[CrossT]

Thanks guys,

I haven't the equipment or knowledge to check/adjust the mixture, and the repair shop checked the injectors about 2k ago [though it can't hurt to try a bottle or two of Techron. So, it sounds like I'll attempt to see if there is a vac leak upstream and the take it to a repair shop. I'll post results/cure assuming there is one.

todd
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86 cpe

[ben parrish]

I had the same problem and the mixture adjustment was the ticket. Don't be scared to try it. There are threads on the subject that help walk you through it. After you adjust it, a reasonable shop should only charge you around $75.00 to check the mixture and make any needed changes.

[gt3r]

I have the same thing with my 85. I didn't think that it was the idle valve, since I could here it humming when the power was one. However, I was tinkering with things during the "hunting idle" situation and I accidently tapped the idle valve. The idle immediately stabilized. I cleaned the valve out but it still hunts on occasion. As soon as I tap it, the idle is fine. Try a gentle tap on the ISV. Good luck.

[Wil Ferch]

If the gentle tap works..then run a few bottles of Techron through...
---Wil Ferch

[john walker's workshop]

first check the charging voltage. those 90 amp alt/reg units are famous for overcharging, and when the voltage swings up to 17, things start to get wierd. hook up a voltmeter and watch it while you raise the rpm. then there's the ones that don't do it all the time. try to check it when the problem is happening.

[CrossT]

I have my fingers crossed that I made some headway on this issue. Used a few bottles of Techron, per Wil's suggestion, and surprisingly, that made a noticeable difference in the overall running [I am a skeptic on these things, so that was a pleasant surprise]. There's a rebate on it now too. I also did a bit more fiddling with the idle speed [bypass] and also blasted out the idle stabilizer vavle with carb cleaner. Between these three, the idle is markedly improved. Feeling a little more confident in my ability to not break anything I touch, I will also get a digital voltmeter and adjust the mixture next weekend. I think it is rich

Thaks John for the comment on the alt/reg. I will look at that next if these items don't do the trick. This car has no electrical gremlins though [I shouldn't have said that . . . ].

Also another thanks to John [indirectly] fixed the passenger side window with Marine Goop in the bottom channel. Working like a champ now.

making progress - thx to all

todd
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86 cpe

[BGCarrera32]

I am doubtful that Techtron did anything for your ISV (or your throttle body for that matter), as its not in the fuel stream. You may have cleaned up your injectors, but that's it.

Could be a reference sensor or speed sensor on the fritz. They are the 2 connections on the same bracket as the CHT connector. I'm fighting a no-start condition right now...thinking my engine has lost its ability to find top dead center, I have no spark or injector pulse either (these sensors look at the flywheel). These are pretty reliable sensors, but it could be possible one goes wacky under temperature (they are located almost above the exhaust crossover pipe.) I'll let you know what I find after tonight, I'm getting some conflicting advice from my meter...

IF the ref sensor or speed sensor began to stumble, it could send confusing or no signal to the DME, so the DME didn't properly collapse the coil, and you'd have a loss of spark and misfire/erratic idle.

Keep in mind I'm speculating here...

-BG

[CrossT]

Interesting thought on the flywheel sensors. Man! How many things contribute to the well being of this system? Another car I have [93 Jaguar XJ6] with a similar ignition system, has such a position sensor that is intermittent, causing a no-start condition with that car. In that car's case, it is the wiring leading to the sensor.

The Techron seems to have made the p-car run noticeably smoother, so I assume my injectors were possibly a bit clogged.

thx
todd
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86 cpe

[Lorenfb]

You might have a fuel mixture problem,
which can cause the idle to "hunt".
Try the following;

1. Disconnect the O2 sensor, and read its voltage
(.05 - .60 volts is normal)
2. Check the fuel pressure (2- 2.5 bars, 30-35psi)
3. Check that fuel is not leaking from the
vacuum line of the fuel pressure regulator,
4. Check that the idle valve is not sticking,
5. Check for intake air leaks with a spray can
of cab cleaner,
6. Make sure that the air screw on the throttle
body is only out about 3 turns,
7. Check the charging voltage,
8. Make sure that no one has adjusted the
air flow meter (It's a key mixture variable.), and
9. Disconnect the throttle switch, the
full throttle contract may closed at idle.

Check out this website, systemsc.com. It has
some good info which may provide further help.

Good luck
Lorenfb@aol.com

[Wil Ferch]

Try more of the carb cleaner through the ISV before any adjustments are made....even though I mentioned Techron through the system, there is a valid point that BG mentioned... that you're dealing mainly in the air-stream. Both actions, however, contribute to good running...

---Wil Ferch ( simple things first)

[scottb]

I had my car in the shop yesterday for a pre-roadtrip "lookover." While it was there the mechanic adjusted the mixture after I complained about the idle hunting problem and told him I had replaced the O2 sensor, the CHT sensor and the ISV. So far (knock wood) the idle hunting problem has been solved. Of course, now that I've gone and publicly said the problem is fixed, it will no doubt return and be worse than before. Ah, the life of a Porsche owner....

[nostatic]

Don't forget to change the oil now. Techron contaminates it, and the conventional wisdom says to do an oil change after finishing the tank of gas that had the Techron added.

[CrossT]

I did not know that about the oil being contaminate by the Techron. Thanks for the note. I don't like the weight of the oil in the systems now anyway, so out it goes.

todd
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86 cpe