Every now an then whilst driving and the car at running temp, if I'm sat at some lights or a junction the engine will start to stutter and stall, a restart starts it up fine and otherwise it runs OK. There aren't any codes so am unsure where to start.

All the usual suspects have been swapped out as I've given the car a major service following swapping out the head gaskets/valve seals (dizzy cap, rotor, leads, plugs. oil, oil filter, fuel filter, O2 etc).

The engine was totally desmogged as there are no emission concerns for a car of this age in the UK.

Any pointers would be appriciated.

Regards

Matthew

 

First:

Do the basics.

Set TPS and IAC per spec here in the Tech Articles.

Then let us know how it goes.

If it still exists after that we can worry about more difficult things

later
Jeremy

 

Cheers, I set both of those not so long back, suppose it's possible that they've moved with all the work that's gone on. The car idles correctly. I'll check them again to be sure.

Matthew

 

maf sensor

 

Have you double and triple checked all ground wires?

One set of three goes to the pass side cylinder head , have to stretch to see it. U said it had a lot of work done. Kinda common to forget to tighten and they have a penchant for coming loose in the first place.

Also is this only happening when the car is hot, up to temp or cold only?

later
Jeremy

 

I desmogged the car, fitted new headers, Y pipe removed the smog pump, cat etc when I had the heads off to do the head gaskets/valve seals. swapped out all the gaskets for the water pump to the top of the intake.

The car drives fine, but if it's been running for an hour and I'm sat at idle at a junction it'll start to stumble, the engine will get erratic and if I don't give it some throttle it'll stall. If I fire the car up after it dies it's fine again, shows no symptoms and there aren't any codes on the ECU (SES light is working, ECU is 12 months old too).

Pretty sure I've not missed any earth cables, did make sure to get them all, will double check.

Matthew

 

Investigate and test:

Injectors
Ignition module
pickup coil

Test them hot and cold. Those are the most common heat related failure pieces on our cars.(grounds tend to get worse as heat increases also)

 

The MAF would trigger a code, he doesn't have any.

 

I don't know if it's relevant but ever since I've had the car it always takes ages to start if the car has been used that day, it'll crank for ages before flooding and spluttering into life (can smell the gas). Always suspected that it could be a leaky injector, I've run various fuel cleaners through but I can't say it's ever made a difference.

 

Drove the car home from work, it's getting really bad, as soon as I come off the gas it starts stumbling, wondering if my pressure reg or pump is on the blink.

Matthew

 

take the injector plugs off one by one and see if you get any difference

 

Most injector cleaners destroy GM injectors of that vintage.

They are most prone to acting up once hot also.

Ohm them out, factory should be at or very near 17ohm,

Below 12 ohm is junk, I recommend replacing as a set anyhow

Any sensor can be bad and not flag a trouble code. This holds true for OBD 1 and OBD 2. They can be skewed out bad enough to be "in range" and cause issues but still be bad from a working standpoint.

later
Jeremy

 

Qube, I'm sorry but I have to ask. You said there are no codes, are you positive that your check engine light is operational? As jamon8 mentioned earlier, this really does sound like the MAF.

 

SES light comes on when I'm starting the car and goes off once it's started. The ECU is fairly new as the old one made the car run so rich I was only getting 2MPG!

I'll meter out the injectors.

Thanks guys for the help so far.

Matthew

 

Certain things will NOT set a code. MAF sensors do NOT go bad as often as people make them out to.

Whay could be bad or on it's way out that will cause the symptoms and won't throw a code is a MAF relay... or a fuel pump.

Persoanlly I'd start be checking the damn fuel pressure, particularly when the symptom manifests.

 

Personally think it's fuel related.

Could the desmog cause it to run too lean on idle?

I'm running a stock PROM.

Matthew

 

when my car started to stumble at random on the road, it turned out to be a bad wire going to the coil, so, that wire was cut, and ive yet to have problems with it.

 

please give us a little more info

like what color was the wire so we know what to look for if our cars start to have the same symptoms

thanks

 

Put a meter on the injectors; all exactly 16 Ohms.

On the coil there's 2 connectors (plus the HT), one goes to the back of the dizzy and th.google.co.uk/e other (white and red wire) connects to the wiring loom near the fuel reg.

The wires look nipped, suspect that they could be broken, are these still available to buy? At the very least I could cut out the bad bits and solder them back together.

 

My mechanic has given this opinion (he's not seen the car yet).

Sounds plausible:

"The stalling problem with no error codes is usually a distributor
Problem. There is a pick up coil around the shaft of the distributor
Which when hot cannot supply enough trigger voltage for the module
And the sparks dwindle.
You need pick up coil, module and in extreme cases a new shaft as this
Looses magnetism."

 

No, the pickup coil will likely throw no code either. Does that mean he is right? :shrug: Can't tell without actually diagnosing and correcting the problem.

 

I don't have the equipment to test fuel pressure so will need to take it to the shop to have that looked at. I'll swap out the coil and dizzy module as they're cheep enough and easy to swap out, if that doesn't resolve it I'll have to take it in.

Cheers

Matthew

 

qube, please post a response once you've gotten the issue resolved - I am currently starting down the same path with my GTA...

 

Any news on this yet??

This is a pretty good thread and should have the answer posted.

 

Hi,

sorry for the lack of update I've been busy so have not had chance to play further with the car.

My parts supplier has given this as a possible culprit:

The most likely cause of your problem is the distributor shaft itself.

The top of the shaft incorporates magnetic material that can become
contaminated over the years - also GM manufactured some of these shafts
with "faulty" material in the first place. Stalling/irratic is the classic
symptom of this problem.

There are a couple of tests you can do do help diagnose this problem:-

A weak magnet can usually be detected with the distributor out of

the vehicle. Simply rotate the shaft by hand; you should feel the
magnetic poles align on a good distributor. Two optional methods of
testing while the distributor is still in the vehicle are:

1) Disconnect the pick-up coil leads from the module. With a voltmeter
set on the AC volt scale and attached to the pick-up coil leads, crank
the engine over and observe the meter. An AC voltage output of .50–.74
minimum should be present. If not, replace the distributor shaft.

2) Disconnect the set timing connector. While observing the timing with
a timing light, increase engine speed to 2,000 rpm. If the timing does
not advance, replace the distributor shaft.


IMO the shaft is less likely to be the culprit than a faulty ignition module, given the difference in price I'm hoping it's the IM.

I'll post some updates this week as I'll have time to investigate further.

Matthew

 

I have to reply with my canned answer about TPI and stalling, since I had the issue awhile back. I inadvertently fixed the problem. Here's what happened, and you should check the same.

I drove the car for quite some time and it was perfect. I drove the normal 1/2-45 min ride on day in the summer, parked the car, and when I went to leave a few hours later- nothing. I finally got it started, but it would not stay running if I let my foot off the gas.

I suspected pretty much everything, so went on to replace a bunch of stuff, including the igntion module. when I changed it, I noticed a ton of green and white crud buildup on the connector of the module, inside the distributor. After cleaninng that all up and swapping the module, it did exactly the same thing. So after getting desparate and trying everything, I figured, let me try the factory ingition module again. after I plugged it in, wha-la! the car was back to perfect.

I later found out that the module sucked that I got as a replacement, (I measure the signal and saw it was not stable), so it turned out that the corrosion on the pickup leads was the problem the whole time, and the sucky module gave the the identical symptoms!!

good luck! try that first, then check EGR, then MAF relays.