need advice before I take car to shop

redcoupe86 · 05-09-2008, 04:24 PM

I have an '86 that Im trying to diagnose an idle problem on.

When I start the car from cold it acts real rough, like its running on 3 cylinders for abuout 20 seconds. Then it revs up smooth to a 1000 rpm or so and trys to hold it to warm up the car, but it cant and it starts hunting. After doing this for abut a 1 minute the idle drops to about 700 rpm and holds that rpm but its rough like the car is about to die. There is a bit of hunting too.

Once the car is fully warmed up the idle smooths out a little more but by no means is it smooth. Idle is erratic and when I tip the throttle in from a dead stop the car acts like its going to die until the rpm get to about 900 +/- the the car accelerates smoothly. Its like its hitting a really lean or rich spot, then it clears up.

I have changed the fuel filter and the ICV valve (I've also put the old ICV back on to test too). I ran throgh all the tests in the Bentley. I have 44 ohms on the two outside terminals and 23 ohms on the inside terminal. I'm also getting 12v on the #4 pin on the connector. Both ICV's hum with ignition on, but when the car is fully warmed up and I take the top hose off (with car off) on the ICV and look down to the valve to see it the has move to half open. My proceedue is to have the car (warmed up) off and I look down to see the position of the valve. I then turn the ignition on and check the valve, but I don't see any movement - I don't think. Its suppose open half way but Im not sure what this is suppose to look like because I don't detect any movement. I did this to the old ICV and then new ICV. Both work (I think), I just changed the old one out not too long ago chasing down another problem unrealted to this situation. I shake both ICV's and the piston clatter. The arrow points down when I install the ICV.

The weird part is when the car is fully warm and I jump B and C (like when your going to set idle speed) the car idles perfectly smooth.

I have the stock chip in right now and it does need to have the mixture set. Heck that could be my whole promblem, but before I do that I want to make sure I have checked all I can so when I go Im not wasting money on a tune.

Any help will be appreciated. BTW, in the past I have checked all the connectors for resistance and voltage and the such and all were good. Not that something could still be wrong there. Also I'm thinking of having them check my fuel pressure just to make sure all it good there.

PCA7GGR · 05-09-2008, 06:48 PM

head temp sensor?

WI wide body · 05-09-2008, 06:56 PM

Had an '84 3.2 that was stuttering and missing after it got warmed up. That turned out to be the O2 sensor...after I wasted a lot of time and #*!#*&#$* words changing the Head Temp. sensor!

However, that doesn't sound like what you describe your engine doing.

hcoles · 05-09-2008, 07:06 PM

not sure but - there was a thread where a guy tracked down that the DME's output electronics that drives the ICV was faulty - the fact that you short B and C and it runs fine makes me that that's not your issue.. but just throwing it out.

also I doubt the ICV will move when the engine is not running. if you have two icv's if you can parallel drive them it might show you something but that may hurt the electronics because the resistance will be 1/2..ok don't do that bad idea..

Anyone (Sal).. does the ICV go back and forth (50% modulation) when B and C are shorted or does it hold steady in the center? I think it modulates.

hcoles · 05-09-2008, 07:07 PM

you can check the head temp sensor.. with an ohm meter I think.. check cold and then hot.
I would need to look in Bentley

redcoupe86 · 05-09-2008, 07:24 PM

Quote:

Originally Posted by hcoles

Anyone (Sal).. does the ICV go back and forth (50% modulation) when B and C are shorted or does it hold steady in the center? I think it modulates.

I dont know if it does that either. The Bentley says that is should open 50% with just the ignition turned to "on". What it doesn't specifically say is that is is "not opened" when the inginition is turned to "off".

BTW, I did repaced my O2 with a "universal" O2 sensor that other pelicans have used.

hcoles · 05-09-2008, 07:38 PM

I think you could try starting from cold with the B/C shorted... it may idle a bit slow or fast but if it is steady and not hunting around then that may be an additional clue to the root cause.
I haven't done this just an idea. Others will likely have better tried and true methods.

ToddM · ToddM's Garage

My best advice, and advice I actually practce before going to any shop - show up pre-lubed and bent over

redcoupe86 · 05-10-2008, 05:51 AM

Quote:

Originally Posted by hcoles

you can check the head temp sensor.. with an ohm meter I think.. check cold and then hot.
I would need to look in Bentley

I just did this but Im going to have to do it again, because I started the car for about 3min. testing if jumping B and C would change anything on a cold start. Answer yes and no. Its still started like it was running on 3 cylinders, but is didn't hunt.

As for the ohm readings, I think I'm doing it right. I set the multimeter to 2k range - the next lowest range was 200. With that car tested about 10 min. after it was stoped read .500k when set on the 2k setting. I think this means its 500 ohms. As the car is cooling the number is increasing. Five min. later is reads .700k. What I dont understand is when I set the multimeter to 200k I get no reading?

When the car is completely cold it should read between 1.4 - 3.6, if it doesn't im reaplacing the CHT sensor.

rllevin · 05-10-2008, 05:58 AM

This is cheap, simple and probably wrong but how's the rubber seal on your oil filler cap? If the seal isn't good you'll have rough idle issues and they'll be worse when it's cold.

jbrinkley · 05-10-2008, 06:08 AM

did your number change? I just tried calling you

redcoupe86 · 05-10-2008, 07:38 AM

Well, I think the CHT is fine.

I do have a question: What sensor tells the ICV how to react - the O2? The reason being is that the car runs smooth with th ICV bypassed. And I don't think its the ICV itself because I'ved tested the old one and new one and the car acts the same.

hcoles · 05-10-2008, 07:54 AM

the ICV runs at different PWM (pulse width modulation) levels as a function of the engine speed. The DME wants to keep the engine at e.g. 880rpm and adjusts the ICV to do that. The engine speed comes from the toothed ring on the flywheel... it sort of looks like a starter ring.. but it is the "speed" ring. The sensor for that comes out right next to the CHT wire. It is one of the three on that bracket. I have a feeling this could be hard to track down quickly.. your mechanic should have a DME he can throw in there to test if the issue is likely in the DME. This should not cost you much.

vash · vash's Garage

Quote:

Originally Posted by ToddM

My best advice, and advice I actually practce before going to any shop - show up pre-lubed and bent over

hahahah! great todd........

redcoupe86 · 05-10-2008, 08:50 AM

Quote:

Originally Posted by hcoles

the ICV runs at different PWM (pulse width modulation) levels as a function of the engine speed. The DME wants to keep the engine at e.g. 880rpm and adjusts the ICV to do that. The engine speed comes from the toothed ring on the flywheel... it sort of looks like a starter ring.. but it is the "speed" ring. The sensor for that comes out right next to the CHT wire. It is one of the three on that bracket. I have a feeling this could be hard to track down quickly.. your mechanic should have a DME he can throw in there to test if the issue is likely in the DME. This should not cost you much.

I wish it could be that simple, but I have already replaced the DME about 4 monthes ago. Not to say that it couldn't to bad, but it's problably fine.

What I do know is that if I unplug the O2 the car runs fine adn the idle does smooth out. This isn't really solving the problem just hidding it. The more I look at all this the more I think just like a previous poster said above - take it to the wrench and bend over! The only problem is that there are no really good Porsche wrenches where I live so I'm left with guys who work all "all german cars" so I'm not 100% confident that they will know all the quirks and procedures of our cars and it will end up costing me and arm and a leg - and maybe and eye!

That's life I suppose.

hcoles · 05-10-2008, 08:59 AM

ok, I didn't remember reading that if O2 unplugged all runs fine... this should be clue.
Sorry there are no good P-car mechanics near you.. how far to drive to one? I don't think there is any issue with driving with O2 unplugged... I've done that for months while I worked on my idle switch adjustment issue trying to smooth out the surging in stop and go type traffic... I got the problem about 80% resolved I think.. I'm now in rear wheel bearing hell so that is on the back burner.

redcoupe86 · 05-10-2008, 09:02 AM

Quote:

Originally Posted by hcoles

ok, I didn't remember reading that if O2 unplugged all runs fine... this should be clue.
Sorry there are no good P-car mechanics near you.. how far to drive to one? I don't think there is any issue with driving with O2 unplugged... I've done that for months while I worked on my idle switch adjustment issue trying to smooth out the surging in stop and go type traffic... I got the problem about 80% resolved I think.. I'm now in rear wheel bearing hell so that is on the back burner.

Its a good 3 hours. I have to go from Louisiana to Texas. I'm hoping that this is just an idle mix issue. Sorry to hear about the rear bearings - that sounds like a real chore!

hcoles · 05-10-2008, 09:06 AM

sorry I can't be of more help.. let us know how it goes...
re. rear bearings - I'm ok... just running into my mistakes and an interesting question regarding the type of bearing I got from our host as replacement. I'm getting good at taking the bearings in and out using my trusty home made $10 puller.