Snoopstah

<center><img src="http://www.alexisbirkill.com/maf.gif"></center><p>Hey,

I'm experiencing a reoccuring issue with my standard 2001 Seat Ibiza Cupra (1.8T VAG engine, 156bhp) that so far the local dealer has been unable to resolve.

Symptoms are intermittant rough idle, lumpy acceleration and, also, when the engine is revved from idle, when it drops back down to idle, the exhaust pops, which could be indicative of a slight misfire/overfuelling? This only occurs when the engine is warm.

In an attempt to figure out what the problem is, and thinking it might be either lambda or MAF-related, I've logged some data from block 02 for analysis during my drive to work this morning. However, I don't really know what I'm looking for, so I'm after some advice!

Despite a rather slow commute into work, I did manage to give it some on occasion, but mainly in first and second gear. Is that going to affect the results?

Anyway, as you can see, the MAF reading peaks at 133 on this particular go. Which, from what I've read, sounds rather low. Is that right? I can confirm that I never got a reading above 138 on the entire journey.

Any other tests I can perform to collaborate this?

Cheers!

QCRAZY

You'll have higher MAF flow in high gears since the added speed "draws" more air in. The MAF numbers you posted look fine.

What does timing look like?? Graph that against RPM if you can.

Snoopstah

<center><img src="http://www.alexisbirkill.com/timing.gif"></center><p>Hey,

Thanks for that. I'll have to try and do a higher-speed run when I get a chance. Will third gear do? - even that will break the speed limit by a fair bit!

Below (above?) is the graph of RPM against ignition timing. What does that show exactly?

(Just to confirm, engine load is reading 100% from 2000rpm above)

Thanks again!

QCRAZY

You want timing to climb as the RPM increases. If the timing went negative (or low) that would show your getting knock and the car is pulling timing back so the engine doesn't detonate. On hot days it may pull timing some but on warm or cool days it should climb. A lot of negative timing is obviously not good. What you have looks good.

2nd or 3rd gear is fine. The key to logging is to do the same gear and the same RPM everytime (i.e. 2000-6500). It's also best to do the same road if you can. The more items you keep constant the better.

Looking at this info and re-reading you post I would wonder if you have a fueling problem (as you said). Does your temp gauge always work?? I can't remember the block (004?) but you shoudl check your coolant temp with VAG.

Is it going up, should settle out around 90C depending on the thermostat in your car?? If its not going up to temp AND/OR your temp gauge isn't working I would look at changing your temp sender.

Sounds like you car is pouring in too much fuel...a bad temp sender is a common reason why.

HTH!

Snoopstah

It does sound like an overfueling problem. Unfortunately, the dealer has already changed the temperature sensor, as the car was at one point overfueling so badly black smoke was coming out!

That appears to have fixed the seriousness of the issue, but it still seems to be affected to some degree. The catalytic converter and front lambda sensor has also been replaced.

Is it possible the rear lambda sensor could be affecting the fueling? I've heard that it only monitors emissions except in exceptional circumstances, but I'm not sure how true that is. Is there a good way to check out the lambda sensors?

Anyway, I'll do a log of the temperature tomorrow morning, and try and do a MAF test at a decent speed. Is there a recommended figure it should reach - if I had a target I could work to...

Thanks so much for your help so far, this car's been driving me up the wall recently!

A.

QCRAZY

Don't know a MAF number for your car. 1.8T owners would have a better guess. Maybe around 140 g/s?

The rear O2 sensor is used all the time after warmed up. Log 031. Should be around 0-0.1 at idle. At WOT it should be between 0-0.9. Let us know what you see.

Snoopstah

<center><img src="http://www.alexisbirkill.com/maf2.gif"></center><p>OK, did a log of temp this morning, and it rose to about 90 degrees in 5 or so minutes. After that, it stayed pretty steady, rising to 98 degrees in slow-moving traffic and dropping to 87 degrees during high-speed driving. So that doesn't seem to be a problem.

I also took the opportunity of being early to take the motorway route, and log some more MAF data. Image shows a full power run in second and third gear, at the end of which I was into three-digit figures on the speedo. Maximum airflow was 132g/s, and peak for the entire journey was 133g/s. Are these figures still reasonable, given the speed being travelled?

Lambda monitoring will have to wait till tomorrow, as I need to register VAG-COM tonight!

Thanks again!

QCRAZY

EDIT: See next post.

QCRAZY

..air flow (thanks A4 guys). It looks like they see closer to 150 g/s of air. However, I think they run a little more HP than your car.

One thing you can do is to unplug the MAF sensor. Then start up the car and drive around. Is the problem gone?? If it's all better replace your MAF. If it's still there look elsewhere.

I don't think this is in your MAF but this would be a good way to rule it out.

Another possibilty is a boost leak. You can run a log of 115 to check that. Compare the specified to actual. They should be very close together. Of course, they'll be a big variance when you spool the turbo's up and when you lift off the throttle. In between thosee two operations they should be within in a few percent of each other.

Snoopstah

Great, thanks yet again! I'll try and get some data logged for that block too. I had considered a boost leak (possibly the crappy standard Bosch DV) but as that wouldn't give the rough idle or misfires/popping, discounted it. However, it might be adding to an existing fault elsewhere.

I think the A4 runs 180bhp - sadly, until I can afford to get mine chipped, I'll have to live with my 156bhp, and be satisfied that my car is lighter )