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760 GLE 2.8 idling problems

Hi All,

I own an '86 760 GLE with 2.8 V6. It used to idle rough, which is why I took it to a local garage here in my village in the Netherlands.

to solve the problem, my garage did the following:

- replace broken air hoses

- replace intake manifold gasket

- replace injector rubbers

- replace distributor rotor and cap

- disabled the EGR (exhaust gas regulation) by stubbing the recirculation pipes. They did this because the exhaust recirculation pipes were rusted and leaked.

- replaced exhaust manifold gaskets

Then, when they reassembled all this, it turned out that by touching the cable harness too often it had really reached its end of life. Fuses kept blowing due to the brittle isolation, which as you may know was a problem in 84/85/86 models due to a bad plastics supplier.

I was able to order a new cable harness (from California), which was an '86 version for a B28F engine. (I have so far still not been able to determine whether mine is a B28E or B28F, normally in Europe you would see B28E's, but this one was a special delivery to Saudi Arabia, when new).

After fitting the new harness, fuses were okay, and the garage wanted to start adjusting the CO2 and idle rpm. Now, after many hours of trying, they admit they failed to do so.

This is a description of the status quo:

- car starts well. idles at 900 rpm if not touching anything

- as soon as I rev the car, engine starts idling at 2000-2200 rpm and refuses to go below that.

- high idling remains even when engine is warm

- noticeable loss of power. Car does not accelerate well.

It seems my car has a constant idle system installed, with a microcontroller controlling an idle control valve. As part of the diagnosis, I ordered another (used, but 'guaranteed to work') idle control valve, which the garage fitted. Car still has the high idle.

Questions I have at this moment, are:

1) is something wrong, or do you think this is now a matter of adjustments? 2) can anything be adjusted on the idle control valve behavior? 3) Could it possibly be a miswiring of the new cable harness? How do I figure that out?

Any pointers are really appreciated!!

Thanks Andre Blum Biezenmortel, the Netherlands

Reply to
Andre Blum
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Just a bit more info:

- the 900 rpm at start climbs up pretty soon even if not touching anything.

- There is a black&white test lead that, when connected to ground, should disable the idle control valve. When I do this, nothing changes. Idle remains high.

- When I disconnect the electric connector from the idle control valve before starting the engine, and then start, nothing changes. The behavior is the same as before.

- my wiring seems to fit this picture: (and not this one:

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idle control valve is wires red-yellow/yellow-grey/white). then, I tried to adjust idle speed myself, following the haynes manual.

- Connected the test wire to ground,

- idle adjustment screw completely in (as this is for non-constant idle systems), other two screws are completely in too.

- Checked throttle cables (that is: I ascertained that they are pretty loose and that the throttle pulley is at its stop).

- put a test light between positive battery terminal and the bottom microswitch connector at the throttle. Ascertained that the light is on.

- next step was to adjust the bottom screw at the microswitch (connected to throttle valve) and adjust to 700 rpm. Could not make this happen.

1500 is what I can achieve when screw is completely out.

Thanks Andre

Reply to
Andre Blum

could come from a leak in the air hose that goes to the idle regulator. Change all air hoses - not only the ones that seems to be broken.

Cheers Per

Reply to
Per Groth Ludvigsen

My thoughts as well - sounds like a vacuum leak - and being an '86 is not OBD2 so diagnosis is not as simple.

Reply to
clare

you seem to have done all the checks an OBDII would have done. i'll go w/ vacuum leak as well, but will point out that vacuum hoses are not the only thing that leaks. if nothing else seems to be working, pressure the entire intake side. this involves some special (easily constructed) widgets, a positive pressure, a pressure gauge & some soapy water.

Reply to
edasduke

Another easy way to check is to feed propane around the suspect areas with an unlit small torch. If engine speed changes (up or down) you have a leak. Real small leaks can be found by spraying WATER on joints.

Reply to
clare

Reply to
edasduke

this might prevent the engine in making a prober vacuum.

Cheers Per

Reply to
Per Groth Ludvigsen

the propane trick. much simpler works very well, has some liability issues. i'm a general contractor from the states. liability is always an issue. i explain to a client how to hang a door (surprisingly not easy, i always thought it was simple) & he/she ends up injured by a lawn mower. it's my fault. just let me say. cold engine, fire suppression @ the ready.

vacuum pressure & positive pressure are different. (something Myst keeps teaching me.) the soap (in the US Joy works best) lowers the surface tension. this produces bubbles.

as a point of interest, newer cars have pressure problems (not your '86) older cars have vacuum issues. vacuum hoses have increasingly been replaced by sensors & icu's.

Reply to
edasduke

Thanks very much to all for replying.

About a week ago, so after posting this, I did indeed check for leaks both by spraying water on the vacuum hoees, and by spraying (flammable) disc brake cleaner in various areas. No change at all in idling speed.

With a friend, I also checked the cold start injector for proper operation and leaks, according to the steps in the Haynes manual. This injector worked fine.

The car was drivable and I took it to another local garage, the boss of which apparently has worked a lot on this type of cars. After spending some time and doing basic checks, he now suspects the idle control unit (ICU). Apparently, it has been messed with in the past (box was open, etc), and with the whole unit disconnected and out of the car, the idling behavior is completely identical, indicating that is does nothing at all. It is on my desk now. No obvious loose soldering or burned components to be seen.

I've managed to locate another used unit, which will be mailed to me, so we'll soon find out.

One thing I forgot to tell is that, when I took the car home and did some basic checks, it turned out that the garage mixed up the wiring of the new cable harness so that the temperature sensor and the wire to the pressure control unit were interchanged. I fixed that. Could it be that this mistake blew up my dear ICU?

I'll Andre

Reply to
Andre Blum

Okay, confirmed that it was the idle control unit (electronics). Idling speed can be adjusted properly now that I have a replacement unit, and though I see some strange behavior still, I believe this is due to adjustments, like of the throttle position switch.

On that subject of the throttle position switch: the haynes manual has ambiguous/conflicting information. In the chapter on idle adjustment it says "connect a test light to lead position 4 of the throttle position switch (orange wire)" On the same page, there is a picture of the switch with its leads 1, 2 and 4, and color codes where orange is on position 2 and green-red is on position 4. When wiring my new harness I took that picture as reference.

Anyway, this week I got a great offer for the car, and sold it.

Once again thanks to all of you for helping me out.

Andre

Reply to
Andre Blum

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