Hello all,

First of all, sorry for the long message. I wanted to give as much detail as possible.

Symptoms:
- Stalling: The car would not be able to hold the revs when they drop as I depress the clutch (while stopping for a traffic light, for example). This seems to be quite common, with many possible causes.

- Hesitation: This happened once or twice while going downhill under "engine braking". As I depressed the gas pedal to accelerate, the car would actually lose power, hesitate, and even slow down. A similar experience is mentioned here .

I searched the archives and read the relevant sections in Adrian's book. I had the caps and rotors less than a year ago. I replaced the battery (it was at least four years old). I also replaced the DME relay. No improvement. I tried to reset the DME by disconnecting the battery and going for a spirited drive, etc. to no avail.

So I took the car to my mechanic. There were no fault codes registered. The idle microswitch and the idle stabilization valve were fine. There were no vacuum leaks. The O2 sensor(s ?) were assumed to be OK given that there were no fault codes.

After not being able to pinpoint the problem, my mechanic ran the scan tool and "re-adapted" the "system". This was not done with the hammer, but with a tool that looks like an industrial-strength tablet computer (PST II). The car was totally transformed. Even the engine note has changed. My mechanic acknowledged the fact that we did not find the root cause of the original problem, and that the problems will probably return. He asked me if I knew if the flywheel was a low-mass one. I have owned the car for more
than three years now, and I have no reason to believe that I have a LWF (but, I'm not certain).

Anyways, after some days of worry-free motoring, the symptoms started to creep back. First, the car started having difficulty catching the revs, then it started stalling. I took the long way to the grocery store yesterday, and the "hesitation" issue was back. It was very predictable going downhill on/ down from Skyline.

What really surprises me is that the car ran perfectly for a while and then "forgot" what it "learned" during the re-adaptation. I would like to do as much troubleshooting as my knowledge and time permits before taking the car in. After all, my time is cheaper than my mechanics, especially when it's raining outside.

Here are the potential leads that I identified:

1. TSB for the air flow sensor connector as discussed here.

My VIN is below the threshold stated in the TSB, so I'll need to check this.

2. Alternator. With the new battery I measured the voltage and got 12.53V with ignition off, after a drive. 13.4V while idling. Now, the second figure is lower than the 13.8V that is cited as normal. This may be the reason for the power cut out, but will it explain why the car ran fine for a while?

3. Old ignition wires. Installing new wires seemed to solve the problem here.

I do not remember if I asked my mechanic to replace the wires during the 90K mile service. I'll need to check the records tomorrow.

Now, am I overlooking any other potential causes? What should I tell my mechanic to look for?

Thanks in advance for your help!

ST

 

Dear ST,
The DME will only log a failed component. It will not find a component which is providing bad information. In this area it is dumb. The system can only detect "no information for X period of time". I hope this makes sense.
I would concentrate my efforts around the airflow sensor and wiring. You need to check that TSB and do the work on the connector if required.
Once that is done I would actually be looking at the condition of the airflow sensor.
Going downhill tends to suggest a mechanical rather than electronic problem hence the hammer not finding a detected logged fault.
Your alternator voltage is too low.
I would still not discount a vacuum problem. May I suggest you find a nice level piece of straight road (with radar etc) and stick the boot in. At between 4000 rpm and 5000 rpm you should feel an extra kick. This is the reasonance valve opening. This is a vaccum solenoid valve system. If this valve is stuck open all the time or if the flapper valve drive arm has become detached it will cause your problems.
Ciao,
Adrian
964C4

 

Dear Adrian,

Very informative, as always. Thanks for the info.

It started raining here yesterday, so it may take me a few days before I can check the leads. I will keep the board posted for future reference.

Cheers!
ST

 

Hello again,

I think I made some good progress on this today. Given that it takes some time for the symptoms to occur after a DME reset, I'm a little hesitant in calling this a success, but I sure hope so.

Last week I made an appointment at my mechanic's and prepared a long list of things that I wanted him to check. On the day that I was supposed to take my car in, there was a big rainstorm here so I called and re-scheduled for this Tuesday.

Today was relatively sunny, so after a few more stalls I decided to see which of the items on the list could be taken care of by myself.

First I cleaned the idle stabilizer valve (ISV) with some carburetor cleaner. It took multiple takes and some time for the cleaner to dry. This was quite easy.

Then I started reading the TSB mentioned above and checked the diagrams to understand where each part was. I had difficulty with the verbal explanations (the terminology etc., made it difficult for me to understand just what the TSB meant). Anyways, I removed the airbox cover and the filter and reached behind the air flow meter to unplug the harness. When I slid the boot off the plug I saw that one of the wires was loose (not crimped down). It was probably still making some contact but it was definitely loose. Here's a picture I took after I separated the wire from the others:



What seemed like an easy fix took hours because I couldn't get the connector out of the plug so that I could re-crimp the wire. After many tries I was able to slide it out a little bit so I was able to attach the wire. At this point I did not continue with the rest of the TSB.

The battery was disconnected during the repairs, so when I put everything together I went for a drive. It seems like the car is running more smoothly and it did not stall on me yet. HOWEVER, it usually takes some time for the symptoms to appear, so I'll probably know in a few days if that loose wire was the only culprit. If so, I'll call my mechanic and cancel the appointment!

Thanks again to Adrian and everybody else who posted on relevant topics. If people were not sharing their experiences and DIYs here I wouldn't find the courage to do simple repairs like this.

Cheers!
ST