RichieRoo

Hi guys. Wonder if you could help me with this problem:

Symptom
For as long as I can remember, after coasting (mostly down hill, no throttle applied), when the throttle is reapplied the engine stumbles, hesitates and occasionally backfires. I’ve seen lots of threads about this, discussing AFM / O2 sensor / coils / leads but I can’t find a definitive solution.

Latest occurrence
About an hour into our journey a few nights ago, the familiar symptoms occurred resulting in an increadibly loud backfire. The car stalled and I coasted to the side of the road.

It restarted ok and there were no nasty metallic noises, however the idle was very lumpy, was reluctant to pull away and smelled strongly of fuel. I had to have the car taken home on a flat bed truck

Checks and observations
When I test the ignition by disconnecting one coil at a time, the car will idle the same on either coil.
I checked for vacuum leaks or any places the induction manifold seal may have separated. All looked fine.
I ran the car for about a minute and then checked the fuel regulator vacuum line. Looked dry.
I expected a host of error codes, but when I hooked it up to my Durametric kit, I only got one – 24 Oxygen sensor short to +/GND. I’ve always had this code even after changing the sensor. The values normally fluctuate between 0-1v as they should.

Changes which have made no difference

• AFM – opened, cleaned and slightly repositioned contact arm. As you can see the tracks were worn:
  
  
  
• Coils – both new Bosch units.
• O2 sensor – fitted new sensor and checked output with Durametric.
• Plugs – Replaced 10,000 miles ago
• Ignition leads – Checked over replaced any suspect leads/ boots.
• Fuel regulator – Just ran the car for about a minute and checked to see if there was fuel in the vacuum line. All dry.
• Intake manifold – recently renewed all rubbers.

So my questions are

1. Any ideas why I keep getting the 24 Oxygen sensor error?
2. Is the track on the AFM so worn that, even though the wiper has been repositioned it could still be causing the problem (would this cause the 24 Oxygen code)?
3. Am I correct in thinking that if it starts / idles ok there can’t be any major valve issues?
4. As a test I was going to disconnect the O2 sensor and jumper the white DME fuel plug, but then in another post I read Adrian Streather advise “Do not go into the DME an start messing with the fuel quality settings.” Is this still a valid test?
5. Does anyone have a definitive cause for the common problem of hesitation after down hill coasting?

Any help gratefully received.

Rich

ras62

Could be the AFM flap sticking, is it smooth and free moving? Is the AFM connector on the rear tied back as per the tech bulletin so stop it touching the loom/pipes at the rear of the engine?

RichieRoo

The flap moves smooth and free. I guess there's a chance it could be sticking in the closed position – hard to monitor when in use. I've even considered a MAF conversion as a replacement AFM is prohibitively expensive.

Yes, the AFM connector is tied up as per the bulltin. All good thoughts though.

ras62

It sounds like the O2 sensor is behind the problem. New sensor or wiring check especially at the connector is a good idea. Has anyone tried to jumper the signal under the passenger seat? Frazerpart do a AFM refurb if you need it.

prschmn

Is the idle switch closing? If not the cars loading up with fuel when coasting.

RichieRoo

ras62 – I guess the new O2 sensor could be bad. Just seems odd that it generates the same fault as the old one and both looked to correctly fluctuate between 0-1v.

The same error code came up with the old O2 sensor in the original cat and the new one in the de-cat.



prschmn – Another good thought, but I've checked the idle switch opens and closes fine (both audibly and with the Durametric). I've adjusted it's position too, but it made no difference. I will no doubt end up replacing it just in case.

RichieRoo

Thanks for that... coincidentally I'd also seen their exchange/refurb offer as your reply came in. I also see that a company called PowerLab do a MAF conversion for a little bit more.

ras62

Some good info on O2 sensors, DME adaption etc....

https://rennlist.com/forums/964-foru...on-please.html

Kappi2907

Check your dizzy belt. Heavy back back-fire can be caused by a broken dizzy belt.

RichieRoo

Thanks for that, it made interesting reading. Very detailed. Learnt quite a bit.

That was my first thought when I was waiting at the roadside, so I disconnected each coil – the car idled the same on either coil. I deduce the belt is intact.

I emailed the photo of my worn AFM to a company who refurb them, the chap was really helpful, I think his summary sums up the condition of the unit:"I am more than happy to take a look at yours but I’m not confident of a favorable outcome." So I'll have to get a replacement (or MAF conversion, although my gut instinct tells me not to add any new unknowns while the car isn't running correctly).

I was going to check the fuel pressure to see if there's any fuel leaking past an injector... Does any one know the thread dimension/pitch of the test port on the fuel rail?

tdiquattro

If you are replacing the AFM wiper area, the cars from the same era have much much cheaper AFMs second hand, eg 944, I suppose you can swap parts from the head accross (the flap is smaller).

I read somewhere that fuel is burned on the overrun/closed throttle to keep heat in the exchangers? (for winter) could this be misbehaving in some way?

ras62

You need to get rid of the code 24 alarm, as that is most likely behind your problem. I would test it with the hammer to monitor the actual voltage from cold as it sounds like the sensor is maybe not heating up due to a wiring issue. Posts 23 and 32 on that thread link explain what should be seen.

John McM

What colour is your exhaust? My car backfires on overrun. I believe I have a faulty FPR as I also have a hot start problem.

FeralComprehension

Hi John,

I just recently started driving my car again after a long maintenance (14 mos) and had a similar issue briefly. I attributed it to a leaky fuel injector as the hot-start exhaust smelt really gassy and (I believe) my mileage plummeted.

I added a bottle of Seafoam to to ~3/4 of a tank of fuel and drove the hell out of it; reran the same procedure using a tank of Redline SL-1. Fingers crossed, but that seems to have helped (only now at the end of the Redline tank).

I know your car has been down on-and-off recently; might be worth a try.

$0.02
fc

John McM

Thanks for the info. My injectors are all relatively new so they should be fine, but who knows with this car? I have a new FPR waiting for install so I'll do that anyway. If it doesn't fix it then a leaking injector could be it. Anything that bleeds pressure is suspect.