It Begins: The S3 1.8T into my '85 Mk2 GTI

[tommymac]

sorry to throw this one out there, but the N75 can cause misfire and misfires on a wideband show a lean reading. is the car misfiring at all under heavy boost?

[tommymac]

also run a manual boost controller and wind it up to about 15 psi or what ever it is your mean to get. My N75 caused mine to go off tap and all over the show. This may help.

I do not think its anything fuel related by the sounds of set up

[Mr_Bob]

Might be a silly question, but it sounds like you're willing to accept anything at the moment!

Is it a rising rate fuel reg?
If so, is it functioning as designed (e.g. vaccuum line blocked, or actuator sticking?)

[GOLFBALLS]

Might be a silly question, but it sounds like you're willing to accept anything at the moment!

Is it a rising rate fuel reg?
If so, is it functioning as designed (e.g. vaccuum line blocked, or actuator sticking?)

Whats a rising rate fuel reg?

I checked the part number in ETKA of the n75 and the one fitted to the car is the correct part. Whether its functioning properly i dont know.

As for the misfire; i'm not really sure as the problem is only really bad under full boost and i'm reluctant to keep revving it up there coz it could destroy the engine!!

The tuner reckons the injectors were only working to around 50% (???) and that there was not likely to be a problem there.

I cant see how there could be a voltage issue with the pump or anything...

[Mr_Bob]

A rising rate fuel reg is generall used for boosted cars.
i'm not an expert on the topic, but my simplified understanding is that it uses manifold vaccuum/pressure to alter the fuel pressure.
this enables it to increase fuel pressure as you come onto boost to prevent leaning out.

so if you have a non-rising rate, or if you have a rising rate with a blocked vaccuum line (or disconnected vaccuum line) this may be limiting your fuel pressure while on boost.

might be worth checking the VDO part number to see if it's rising rate or not?

[tommymac]

my bets on the N75 and the misfire causing a lean reading. Its really worth running a manual controller for a run to see if does it. mine went mental and was all over the place. I guess you are running cold plugs bk7? I am just thinking that the reading is incorrect and it could all be over a minor issue. if you have any misfire on a dyno it will send the A/R to silly levels like 20-1 and even higher.

[GOLFBALLS]

A rising rate fuel reg is generall used for boosted cars.
i'm not an expert on the topic, but my simplified understanding is that it uses manifold vaccuum/pressure to alter the fuel pressure.
this enables it to increase fuel pressure as you come onto boost to prevent leaning out.

so if you have a non-rising rate, or if you have a rising rate with a blocked vaccuum line (or disconnected vaccuum line) this may be limiting your fuel pressure while on boost.

might be worth checking the VDO part number to see if it's rising rate or not?

Ah ok. Yea its definatley the right fpr for the AMk engine and all the vac lines are new and clear.

I will chase up the n75 theory though tommy, thanks for that!!

Cheers,
Wade

[GOLFBALLS]

my bets on the N75 and the misfire causing a lean reading. Its really worth running a manual controller for a run to see if does it. mine went mental and was all over the place. I guess you are running cold plugs bk7? I am just thinking that the reading is incorrect and it could all be over a minor issue. if you have any misfire on a dyno it will send the A/R to silly levels like 20-1 and even higher.

Just spoke to the tuner agian. He reckons it wasn't really misfiring, and normally that will make the reading rich (fuel not getting burnt).

He mentioned the fact that it is unlikely - but possible - that the timing may be out at high revs/peak boost... now, this may seem like a really stupid question, but these engines normally have variable valve timing don't they? And if so, the system that senses this function is disabled with the qpeng setup. Could something be amiss here, or is that system only there to shut the engine down to avoid a catastrophe if it senses a problem?

I changed the belt myself before installing the engine and i'm pretty sure i did it right, but maybe i got a tooth out??

[Jarred]

I read that there was a plate or something you had to make up (or buy) to convert VVT into non VVT.

There might be something in that. I can't remember which engines had VVT and which ones didn't.

That wouldn't explain why it still works on low boost at high RPM though? It's only possibly mis-timed at high revs under boost...?

Just spit balling here mate

[tommymac]

odd i know for a fact misfires read lean not rich - the fuell is unburnt so there can be no rich reading at all. I am sure these guys know what they are doing but thats basic, search the net mate its everywhere. Try the N valve anyway.

AMK has VVT

"An extended range oxygen sensor can “read” air/fuel mixtures as rich as 9 to 1 and it can read air/fuel mixtures as lean as 19 to 1 (a standard oxygen sensor is only accurate at air/fuel mixtures of around 14.7 to 1). This method has the advantage of extremely fast reaction times for the readings, but false lean readings can be created under certain operating conditions. The false readings are created when the oxygen sensor “sees” the excessive oxygen in the exhaust system that can come from a engine misfire, a supercharged engines “blow-thru” effect (at low engine speeds), a high performance camshaft’s valve overlap or an exhaust leak."