Newbie questions

[1bad914]

I am new to 911’s, I have owned a lot of VW’s and about every other Porsche out there but a 911. I bought a 1984 ROW 911 Carrera about 16 months ago; it had 82k miles on it and had some decent mechanical records. The paint was tired, so I tore it all down and painted it. I put it all back together and drove it most of last summer. Okay enough history, I have four problems that are driving me crazy:
1. Hunting idle when cold, I have read all the info I can on this and have cleaned the ICV and set the idle, it still goes up and down when cold.
2. Rough idle when warm, once it is warm and stops the bouncing idle it idles rough, I don’t think that I should be able to feel the car shake while at a light. You can see the tach bounce.
3. It cuts out while driving, I will be driving along and it will just drop power, it comes right back up. It is like it hiccups.
4. Hesitation at minimal throttle, it seems to be around 2200-2300 rpm, but you will be in third and just cruising and it starts to jerk or hesitate, I have to hit the gas or put the clutch in to stop it. Usually I just shift into 4th and it stops, but it annoying. If I am into the throttle it does not do it, only when I am just barely on the gas. It reminds me of a carbureted car that has a gap between the idle and main jets.

I love this car, but man is it driving me crazy. Any advice, beside start all over.

[twistoffat]

The Idle could still mean the Idle control valve needs replacement but the cuttiong out while driving suggests a DME relay problem. Try change the relay and see if it that works

[jmohn]

It's been some years since I had a Motronic system car ('86 951, essentially the same system), but the symptoms you're describing are classic for a Motronic sytem that just needs a thorough"tune-up". The idle problems and the cutting out\hesitating problems may or may not be related. Assuming the plugs, plug wires, dist. cap and rotor are good, I would focus on the idle control switch adjustment (for the idle issues) and the air flow sensor (for the hesitation\cutting out problem), the "traces" in that mass air box can become worn; also fuel supply pressure and pressure regulator. If you can find it, the repair manual has a logical procedure for checking the Motronic system (I have it for the 951 but not the 911) that should be followed so you're not adjusting\replacing the wrong thing. Hopefully someone can point you to that procedure either here or elsewhere on the web.

One final thought, be sure you have good, "hot" battery, good charging and good grounds through-out the car, the Motronic system is very susceptible to marginal voltages which can cause a variety of starting and\or drivability problems.


Good luck,

Jerry M
'78 SC

[1bad914]

Thanks for the advice, I will attempt a few of these things. The first thing I am going to do is adjust the valves and do a full tune-up, then I will have a good baseline.

[jmohn]

Here's a trouble-shooting chart that may help:

Pelican Parts: 911 Tech Info - DME Troubleshooing



Jerry M
'78 SC

[Justin@Athens]

I'd suggest you have multiple small vacuum leaks, at least that is what I am dealing with and have the same kind of symptoms.

[kidrock]

Originally Posted by Justin@Athens

I'd suggest you have multiple small vacuum leaks, at least that is what I am dealing with and have the same kind of symptoms.

+1...a bad O2 sensor and/or rich mixture will create the hunting idle issue as well.

[1bad914]

Well, I am believing that the 02 sensor might be part of it, the weather has turned warm and it is amazing how much better it is running, it idles a little higher, but no shakey idle. hmmmmm I will dig in soon and baseline the car, then really find out what id going on.

[Talewinds]

You're going to get A LOT of recommendations here about how to address your issues since many of us have experienced the same problems with different causes, apparently.
ICV
DME
Vacuum leaks
Fuel pressure

I was experiencing exactly what you described and here's what I did:
1) ICV removed and cleaned, verified operation. No change
2) Fuel pressure, verified. No change
3) Vacuum leaks, searched for, found none. No Change

4) Idle position switch. Was not making contact when throttle was closed at idle speed, adjusted. I did this activity in conjunction with #5.
5) Replace O2 sensor and then use it along w/ a voltmeter to set my fuel/air mixture. Discovered it was too rich. Adjusted properly.

The car started, ran, and idled PERFECTLY after that. Night and day different.

[RWebb]

Originally Posted by twistoffat

The Idle could still mean the Idle control valve needs replacement but the cuttiong out while driving suggests a DME relay problem. Try change the relay and see if it that works

check all the grounds too - esp. the strap on the trans.

[porschenut]

Dirty injectors? Old spark plugs? CHT sensor?

[1bad914]

Talewinds, how did you accomplish #5 in your list, you mentioned that you adjusted it with a voltmeter?

I am adjusting the valves today, and will put new plugs in along with a new cap and rotor. The wire are fairly new. I think I will just order a new o2 sensor.

[slimlynn1]

Not sure about your 84 ROW but my 85 ROW does not have an O2 sensor, I would check before you get one.

[E Sully]

If not the O2 sensor or vacuum leak, this link explains the AFM and how to check it with a 9v battery. 911 operates the same as the 944 example.
Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk

[Talewinds]

That AFM link above is a good one, but I wouldn't want to go digging into that thing until I've exhausted some of the more easily diagnosable issues.
Slimlynn brings up a good point, check and see if you have an O2 sensor first.

Then, here are some instructions for adjusting your air/fuel ratio (AFR) provided by DRACO in the "idle bounce" thread on page 4. In that thread you can read all of my posts starting with my identification of the problem on about page 3 then leading to the solution on pages 4-5. There are some great and worthwhile contributions in that thread by other members, I'd like to name names, however I'd probably leave someone out inadvertently. Suffice it to say that thread has become the mainstay for AFR and starting/idling diagnosis issues due to some wonderfully technical contributions from other members.
So here's the poop:

Bring her up to normal temp.

Get a DVM (voltmeter) with a Alligator Clamp for the Red Lead. Disconnect the O2 Sensor Harness and connect the Red Lead with Clamp to the terminal that reads the O2 Sensor at the Cat.

Connect the Black Lead of the DVM to a known good ground. Top of the ICV is a good one.

The reading you want to see is between 0.20-0.60 volts to proximate 14.7 AFR at idle (no higher than 0.80 volts, more than this is too rich).

The AFR adjustment port is underneath and to right rear on the AFM. You need a 3MM Allen Wrench or this tool:

Some other things I might add; The port for the AFR adjustment might have a yellow plastic cap covering that allen adjuster so pop that off to get to it.
You can leave the car running and connect the voltmeter (in a way that the leads won't get yanked into the pulleys!). Then you can make the AFR adjustments with the allen wrench and actually watch the results on the voltmeter with each turn of the allen screw!