BLK964

Hi everyone,

My car has a frequent hesitation problem and occasionally followed by a pop sound when starting to accelerate. It sounds like a pressure release in the exhaust. Usually after a second or two (or it pops) the power will kick back in and it will be fine. It also does this when slowing to turn or when you get back on the throttle after coming down a hill. When this isn’t happening the car seems to run fine and the engine pulls strong.

The possible culprits that I know of from searching previous posts are: Low voltage, Distributer belt, ignition system, and possibly the ISV or airflow meter. Is this the order you would use to trace the problem?

Battery, alternator and regulator are new. Voltage varies from 13.4 to 13.8.

The distributer drive belt was changed by the PO a couple of years ago, however the vent kit was not installed. I have the vent kit and will be adding it, but I will be checking the condition of the current belt. Has anyone ever had to replace the belt more than once?

I have new caps, rotors and plugs on the car. I’m assuming I need to check the coils and plug wires after checking the drive belt.

Any other possibilities or things I need to check?

I have already checked the airflow meter harness for the TSB. It didn’t have a black dot, but it was zip tied. I cut off the zip tie and checked it anyway, then replaced the zip tie. I’ll be putting the black dot on to save any future owner the trouble.

Thanks for your help!

gilessav

Hi Jason,

I had some hesistation issues a while back and had the dist caps/rotors/belt etc replaced and also the 02 sensor as it was logging a fault and can cause rough running and idle. Do you have a stable idle or does it cycle high and low. This seemed to cure the problem for a while but it returned. I then had the car serviced (24k) with new plugs and has the airflow meter investigated. Problem seemingly resolved now and my mech thought it was likely to be the airflow meter connector (92 car) which may have been corroded. Says he's seen similar before. Clean up of connector, reset of DME with Hammer and alls well it seems. I may require a new AFM in the future as the tracks get pitted and this part seems to be associated with a whole array of running/idle/powerloss problems. There was a thread on here ages ago telling of how to open up the meter itself and clean the tracks etc with pics but dont have the link i'm afraid. A search may reveal it if you dont have it already.

Hope you find the cause as i know how frustrating it is, you sound on the right track though with your investigations to date.

Cheers
Giles.

BLK964

Thanks Giles.

I had considered the O2 sensor, but the idle appears stable at the moment. I won't dismiss the thought. Thanks for the lead on the AFM thread. I'll see if I can find it. Hopefully this will be within my low DIY capabilities.

Thanks again.

gilessav

No problems,

the link i was talking about is titled as below from the 964 forum and gives pics etc of all the procedures. Check it out as may help. I personally bottled it and let the specialist do his stuff.

ISV, air-flow meter and idle microswitch clean

springer3

Jason:

The first thing to do is check the compression. There is no way to get the engine to run correctly if compression is down on one or more cylinders. You will get symptoms for non-problems, and spend a lot of money fixing things that are fine. I learned this the hard way many years ago, fortunately on an engine much cheaper to buy parts for. If compression is good, the problem will be minor, and just needs to be tracked down by process of elimination.

Your engine needs compression, clean fuel, clean air, spark, controls, and a clear exhaust system to run perfectly. I believe in confirming compression first, but after that, go with the cheap and easy item first. Work your way up to the more difficult and expensive as you eliminate the easy problems.

I can help you get started if you want to make another trip to my place. A good first place to look is the fuel system. If the filter is old, replace it. Cut the old one open with a hack saw. if you see water or goo, the tank needs cleaning.

The injection system needs the correct pressure, and must not leak down. Leaking injectors could be causing your symptoms. Checking fuel pressure and leak down is easy on most cars, but I don't know how it is done on the 964 yet. You are using brand-name premium gas aren't you? A bottle of Techron is useful to try and clean the injectors if cheap fuel is the problem.

If the fuel injection checks out, the ignition system is the next suspect. I have a home-built ignition scope that gives ignition traces identical to a Sun machine. Come by if you want to have a look at the patterns on the two distributors. If you have dirty plugs, fuel problems, bad coils, or insulation breakdown, that shows up in the ignition trace. My 92 runs like a top after the recent tune up, so we will have a second set of patterns to look at to tell if your's are amiss. Eliminate injection and ignition problems, and all that remains is controls, air intake, and exhaust. I recall the Hammer was showing the knock sensors and ignition timing were operating fine when the problem was happening. We did not confirm the injectors were pulsing, but the background noise may have been the problem.

We can put my oscilloscope on the air flow signal. If the track is dirty, that will cause noise. This does not sound like ISV, as it is closed when you are at speed.

All this should take no more than an hour or two. If the problem is not located by then, you have a strange one. Just remember, there are a finite number of of components in the car, and it is just a matter of systematically trying your hunches until the problem reveals itself.

It is ironic that in 2 1/2 years, my 964 has never mis-behaved. I am definitely not complaining, but I have all the necessary toys, and only get to play with them on the family's "economy" cars. 964s are really quite reliable if you keep after the small stuff. Your's will be too once you get this sorted out.

Good luck!

BLK964

Thanks for the offer. Just PM, email or call me and let me know when you're available. I'll make myself available.

BLK964

Well, my hesitation & misfiring problems have been sorted.

After my last post, my car was quickly getting worse. On top of my hesitation and misfiring issues, it developed a rough idle and started having cold start issues.

After checking out the distributer drive belt, I met up with springer and we checked out the ignition system on his ignition scope. Both coils checked out fine. We also checked the fuel pressure and delivery, which were also fine.

I had 3 DME faults showing on the hammer... one for running rich, one for the head temperature sensor and one for the oxygen sensor. The voltage level on the O2 sensor was low causing it's fault and the rich running condition. The head temperature sensor didn't appear to give accurate readings initially (i.e.- no fluctuation). Later we took it for another drive and it started fluctuating as you would expect. I decided to replace it along with the oxygen sensor, since I was having cold start issues, it was recording a fault and I didn't trust it's reading.

I changed out both sensors and the car is running great.

The oxygen sensor tool I ordered should be getting here soon, if anybody needs one. Unfortunately, I did find that one of my inner CV boots was busted, so I've ordered new ones.

Many thanks to springer for his help sorting out my issues.

Now for the fun stuff... I'm finally working on fitting my brake ducts and RSR splitters and I test fitted an MAShaw RSA spoiler yesterday. I want to try to get these to the shop to be painted next week.

I'm currently looking for the best place to get Bilsteins and H&R springs, if anyone has priced these recently.

Somebody said there was a slope around here, somewhere...

garrett376

Jason, thanks for posting your valuable followup!

Enjoy the slippery slope!

springer3

The default used by the DME if the CHT sensor is bad is 255 degrees C. We cleared the code, and the CHT temperature readings started climbing after starting the engine. Thinking about it later, it was the DME simply counting up to the 255 degree default value, because it was not getting any data. I am sure the CHT sensor was dead.

It was fun getting to troubleshoot. Better still, I got to play with other things while Jason did the dirty work.

We did not trust the hammer at first, and instead did troubleshooting the old fashioned way - checked everything in order of increasing difficulty, and identified the problems by process of elimination. It was interesting that the hammer had it exactly right, and had no false alarms - the O2 sensor the the CHT sensor were bad, and there were no other problems. Compression, ignition and fuel injection all checked out fine. Bad data input to the DME was causing the problems.

Jason has eight or nine climate control fault codes. I had five on mine. Next project will be to get all those sorted.

Dave R.

Interesting thread, thanks for the reports.

Springer, 255 C works out to about 490 F which is very hot.

Are you sure 255 C is the default?

springer3

With a failed CHT sensor, the hammer listed a constant 255 C as the head temperature. Defaulting to the hottest head temperature is reasonable, because that ensures conservative settings of the engine parameters. I am not even certain if the DME uses CHT for engine control. Perhaps someone that knows more than I can clarify.

The DME does use the CHT to enable cold enrichment for easy starting and better running on cold days. The failed sensor was the reason Jason was having starting problems.

RichieRoo

Hi chaps

Revisiting this old thread as my symptoms are exactly the same as BLK964 describes in his first post.

How tricky is it to get to the cylinder head temperature sensor. Is there a good write-up on the removal/installation anywhere? (did have a good search, but couldn't find anything)

Thanks

Rich

boxsey911

Rich, this should help you:

https://rennlist.com/forums/964-foru...ad+temperature

RichieRoo

Cheers Steve

I did come across that thread, but I was wondering how difficult it is to
remove the sensor from beyond the grommet which Jason highlights with
the blue arrow (see pic).

I read somewhere that you really need a special tool to get the CHT sensor
out / reinstalled.

RichieRoo

OK, this is odd. I've been out to check the car over in the freezing cold.

Checked the CHT sensor - at the white (yellowish) two pin connector it gives
a resistance of 5,000Ω which is a bit outside Loren's range of Cold ~
2,000-3,000Ω but then it is -4°c tonight, so it sounds about right to me.

However, when I reconnect the connector (or ever bridge the two wires)
I get an open circuit at the DME connector (testing pins 45 and 48).
PS. These turned out to be the wrong pins, they should have been 45 and 30.

So a few questions:
1) Does the test I am doing look correct?
2) If the wires running from the sensor to the DME connector are damaged
where should I start looking?
3) Whats that loose plug doing at the back of the engine bay. Should it be
plugged in somewhere?

Rich