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[Jay-C]

Thanks for reply Kevin.

I had lifted my foot from throttle, but my car has autobox and this time the engine was not braking either. Do you have any tips how to check the pipes, seals and the breather box vacuum "bulletproof"?

[Kevin Wood]

Ahh, OK. Of course, it's throttle position, not necessarily pedal position.

I might start by measuring the vacuum at the "T" piece by the brake servo. Use a gunsons "Low gauge" or similar. 18-20 In.Hg would be a typical reading at idle when warm. Any less and it might indicate a vacuum leak somewhere.

Kevin

[Jay-C]

Hi!

The codes are still on. Now I have had a smoke test for vacuum leaks, and according to the mechanic my car has no vacuum leaks at all. He also tested the lambdas and they were ok. When cruising downhill and the pedal is up and throttle is light, both STFT's rockets up to 25. Here is some recent data:



Anyone got ideas how to proceed? Could this be caused by faulty ignition coils?

[feeutfo]

No, coil packs won't bother it.

Usual issue with fuel trim codes is maf, swap yours for a known good one and see if it behaves to diagnose. If faulty search the Bosch part number on the maf itself, it fits numerous vehicles including the 2.6.

[webby23]

Totally agree, my 2.6 showed 0170 and 0173 and maf sourced off ebay for £22 quid (sensor only)

 

[Jay-C]

Thanks, I've already changed the maf for a brand new OEM Bosch part (same part number) from vauxhallworldparts.co.uk

In the most cases I've seen, the long term fuel trim values are high if the maf is faulty. In my case, the LTFT values are low but short term trims are equally high.

I'm starting to doubt the smoke test, is it bulletproof way to check the vacuum leaks?

[feeutfo]

Thanks, I've already changed the maf for a brand new OEM Bosch part (same part number) from vauxhallworldparts.co.uk

In the most cases I've seen, the long term fuel trim values are high if the maf is faulty. In my case, the LTFT values are low but short term trims are equally high.

I'm starting to doubt the smoke test, is it bulletproof way to check the vacuum leaks?

Never heard of it. A pudding pull IMO.

[feeutfo]

Where are you in the world? Roughly?

[Jay-C]

Well, I assume the smoke tester is something like this: http://www.youtube.com/watch?v=FfiQ7qWiG-M

I wasn't watching the test in the shop, but it was Bosch car service...

[feeutfo]

Load of 'dangle berries'. Intake is negative pressure times a lot more than the positive pressure supplied there. Not a definitive test by a long way

[Kevin Wood]

Considering what happens with light versus full throttle for a minute:

Light throttle : Vacuum in the intake system post-throttle is high, as the engine is sucking against a closed throttle. Airflow is low. Very little vacuum in system up to the throttle due to low airflow through MAF, air filter, ducting, etc.

Full Throttle : Slight (much lower) vacuum in the whole intake system from air box to intake valve due to high rate of airflow.

What we are seeing is high positive fuel trim in the Light throttle scenario and normal trim under heavier throttle. Positive fuel trim means fuel is being added ergo there is more air getting into the engine than the MAF is reporting. This means an air leak.

As it is not happening under heavier load, we can infer that it's the part that's under vacuum at the time that's causing the problem, i.e. the intake system after the throttle so plenum and manifold seals, Vacuum supply to brake servo and multirams, bung where the ICV connects to the plenum, breather bridge seal, etc.

I wouldn't put a lot of faith in a smoke test either. Better to give the above a thorough inspection for damage. Maybe go round using a length of hose to the ear as a stethoscope or spray some carb cleaner over the areas concerned and see if it upsets the idle.

Also worth putting a vacuum gauge on the T piece by the brake servo to check the vacuum at idle. 18-20in Hg / 30 kPa would be about right. If it's less it might confirm the suspicions of a leak.

Kevin

[Jay-C]

Much appreciated the clear explanation Kevin!

I just ordered the Gunsons G4103 LoGauge vacuum meter from ebay, so I'll try to find the possible leak myself. It looks like the leak should be equal to both sides of V (from the equal STFT1&2 trim values).

[Kevin Wood]

Much appreciated the clear explanation Kevin!

I just ordered the Gunsons G4103 LoGauge vacuum meter from ebay, so I'll try to find the possible leak myself. It looks like the leak should be equal to both sides of V (from the equal STFT1&2 trim values).

Yep, another good point, so something common to both banks.

Kevin

[Jay-C]

Finally got the Gunson meter, here are the idle results, taken directly from the vacuum line from intake system:



That doesn't look too bad?

[Jay-C]

Just wanted to feedback the happy ending: There was a small vacuum leak in one of the vacuum tubes. After fixing this leak by replacing the tube, there has not been any error codes.