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View Full Version : My car is denying me WOT


SnakeyesTx
10-16-2008, 03:15 PM
What a bizarre thing to have. Picture this if you will :

You're at a red light, air is cool and dense, motor runs good, throttle response is nice and snappy. You turn off the electronic nanny known as ASC, sitting in first gear you build up a little rev, light begins to burn green. Your animal instinct reacts as you dump the clutch and give the throttle the beans. Your car launches fairly hard, you feel that pleasurable seat sinking experience and you hang on for the ride. Rev's reach close to 4500 and climbing fast and you start to hear it.... bang! Backfiring. You back off and you're in second at half throttle just driving nice and normal in defeat... your simple cave-man pleasure ruined by the one loud POW or series of smaller pop's in the exhaust.

...so my lovely little `97 540/6-speed has no error codes. Once in a blue moon I get a MAF error but it usually goes away within a minute or two. I took the MAF out, cleaned it, and now the light almost never comes on. But, now I get the backfire at WOT. Why?! WHY!!?!??!

Any ideas where I should start looking first? =/
filip00
10-16-2008, 04:55 PM
i'd try to use the brake cleaner on MAF again...
Rus
10-16-2008, 06:48 PM
Have you had the crankshaft position sensor replaced? I've read reports of some aftermarket sensors causing backfiring. The remedy was to install an OEM sensor. Apparently aftermarket sensors aren't always made to the correct spec/form.
jofusfarr
10-16-2008, 08:56 PM
I had similar symptoms when I had a weak fuel pump - might want to check fuel pressure to ensure it's up near ~50PSI.
632 Regal
10-16-2008, 10:10 PM
old school rule, most still applies: backfire through intake fuel issue, backfire through exhaust electrical issue.
Ferret
10-17-2008, 04:35 AM
old school rule, most still applies: backfire through intake fuel issue, backfire through exhaust electrical issue.

+1

It's probably something to do with the sparkplugs or your crank sensor - though the ECU having no codes probably rules out the crank sensor.

Pull your plugs/leads and have a look - if your plugs have got the brown ring of death on them... you've got HT leakage.
Tiger
10-17-2008, 08:50 AM
Please use the right cleaner for MAF... other cleaner will destroy the MAF... melting plastic... which would ruin the wire to the sensor.
SnakeyesTx
10-17-2008, 08:56 AM
I did put new plugs in about two months ago, pulled them about a month ago, and shockingly enough, one or two plugs ceramic looked cooked already. They were slightly yellowed around the base "nut" part and there was what almost looked like stress cracks all around them already. I've suspected these to be the problem before, but never really knew what the heck would cause it to burn them out so quickly!

I'll try to pull them and show pics this evening, weather permitting.

As for the MAF cleaning, I did buy MAF-specific cleaner, not just generic stuff. I think since then that's what stopped the occasional SES light.
SnakeyesTx
10-18-2008, 11:02 AM
Update : Looks pretty cooked to me... all 8, and they're only about 4 months old! WTF? :(

http://i98.photobucket.com/albums/l273/Tenki-Kun/IMG_0126.jpg
filip00
10-18-2008, 03:08 PM
how many miles have you had 'em?

last ones i had lasted only 14k miles!
SnakeyesTx
10-18-2008, 04:11 PM
how many miles have you had 'em?

last ones i had lasted only 14k miles!


... about 2500 miles :D

NGK used to be very good to me, till I got into bimmers. My E34 cooked 1 occasionally here and there, but my M62 just ate them in one fell swoop.

Scary huh? I just splurged and bought those new Bosch Iridium +4's number 4503's. Car feels like a rocket again.

Edit : New sig pic, finally
Ferret
10-20-2008, 05:01 AM
... about 2500 miles :D

NGK used to be very good to me, till I got into bimmers. My E34 cooked 1 occasionally here and there, but my M62 just ate them in one fell swoop.

Scary huh? I just splurged and bought those new Bosch Iridium +4's number 4503's. Car feels like a rocket again.

Edit : New sig pic, finally

Seriously, when was the last time the spark plug leads were done? This is a classic symptom of that - your new plugs will be cooked in the same mileage!
Tiger
10-20-2008, 08:38 AM
Nice wheels! That spark plug picture is the typical BROD... Brown Ring of Death... you need to change out the spark plug coil leads.
SnakeyesTx
10-20-2008, 09:00 AM
Plug leads? I've got the coil on top of plug setup like the E34 had (with a small boot leading down to the tops of each plug). I'd imagine if all 8 were bad, I'd have some kind of errors wouldn't you?
Ferret
10-20-2008, 11:43 AM
Plug leads? I've got the coil on top of plug setup like the E34 had (with a small boot leading down to the tops of each plug). I'd imagine if all 8 were bad, I'd have some kind of errors wouldn't you?

By plug leads I meant plug boots, and no when 6 out of 8 of mine were shot the car was only throwing the occasional misfire code.
SnakeyesTx
10-20-2008, 03:23 PM
By plug leads I meant plug boots, and no when 6 out of 8 of mine were shot the car was only throwing the occasional misfire code.

Ah okay, so the ends are changable... wasn't sure. I'll definitely look into it because I figured if I had to change all 8 coils, I might as well go for those BavAuto upgraded ones for a few bucks more.
632 Regal
10-20-2008, 09:40 PM
upgrade plug boots??

I might as well go for those BavAuto upgraded ones for a few bucks more.
SnakeyesTx
10-20-2008, 11:38 PM
No, not just the boots, BavAuto has upgraded coils, leads included; but the set is about 600 bucks (as opposed to 550 or so from the local parts houses for the OEM spec ones). Didn't know the boots were removable but if that's all that's going on, then I may dodge a bullet and save a lot of money.
Ferret
10-21-2008, 04:29 AM
No, not just the boots, BavAuto has upgraded coils, leads included; but the set is about 600 bucks (as opposed to 550 or so from the local parts houses for the OEM spec ones). Didn't know the boots were removable but if that's all that's going on, then I may dodge a bullet and save a lot of money.

Yeah, to remove the boots - pull the whole assy off the car, and clean any oil off the spark plug boots.

Grasp the boot firmly in one hand and the coil pack in the other hand, twist and pull apart at the same time. The rubber will try to cling to the coil pack and will eventually release with a snap.

Scuse the valve cover gasket:
http://i11.photobucket.com/albums/a190/Ferret101/Car/IMG_1707.jpg

The new ones are about £7 each from the dealer I think...