Back fires rough idle after filling up

[rmahnovetsky]

With a doubt every single time I go to the petrol station and fill up with whatever amount I always get rough idling and my car backfires like crazy when I'm off the throttle.

I get rough idling from time to time but when I really notice it is on humid days. Sometimes my car just will not idle after I let go of the throttle, I would have to gently bring it to idle.

It is driving me nuts!!!

Belive me I have been under the hood checking for leaks, testing the electronics pulling apart and cleaning.. Im missing something...

But there is one thing I noticed. On my safc I noticed when my idle plays around my airflow readings is different instead of idling at 750 rpm with afm reading of 0.9 it would be 1.1-1.4 ... Im thinking maybe my afm is effected some how by humidity but if does not explain the filling up problem. My guess atm is maybe the fuel regulator or pump but I'm not going to spend $$$ on a hunch. Has anyone go any ideas...

btw my safc is tuned for 4500 revs and above so this has no effect.

[Trust33]

set the throttle position screw a little more in,so it lets a little more air in the motor.

in other words the butterfly might not be letting enuf air into your motor when the throttle is shut.

you will more than likely have a higher idle speed though.but let the engine get the air it needs

Just open it up slightly and check it out it's an easy fix to just in case it has to be set back to stock.All you need to do is mark it's origional location with a marker of some sort.

Try it and see what happens.

It is hard to see but the orange writing in the pic says this is the screw and the blue arrow points to it..

It wont fix the back fire problem though.

[The Dan]

Is it possible that when you fill up it stirs up crap sitting in the bottom of your tank and gets sucked through the filter. Check your fuel filter and remove the pump and inspect the bottom of the tank. Won't cost you anything.

[rmahnovetsky]

Trust33 : thanks. I have increased my idle to sit ~ 850. It is better now. But idle still stuffs around when I fill the tank - everytime. The car came out of the factory idling at 650 so this is a nice patch for now until I find the real problem.

3LIT3 FORS3: I have changed the filter not that long ago. It made no difference

I tried getting the pump out. My good the screw tight!!!! So I gave up.. Any suggestions on getting the pump out?

tikki169 Told me one of his friends has the some problem, but I dont kow how it was fixed yet. So there must be someone else out there that has had this problem and is now solved.

[The Dan]

I would like to help you out. Where are you in Brisbane?

[rmahnovetsky]

I live in indooroopilly. PMed

[rmahnovetsky]

On the weekend I went for a cruise to see a few mechanics to ask their option on my idle problem.. One worth mentioning is the guy from bayside performance. Nice guy seems honest and did not cost a thing - just good old aussie advice. Here is what he said.

Raf: I have a idling problem every single time I fill up my tank with petrol.

BP: Is it worse in hot weather

Raf: Yeap

BP: Does it happen any other time

Raf: no not really

BP: mmmm I reckon it is the fuel cap not releasing enough pressure...

Raf: Why?

Bp: you fill up the tank, when you run the car the pressure builds up in the tank thus increasing the pressure going to the fuel rails. Then when you release the cap fill up with petrol the pressure has dropped and you computer has to re-adjust itself to correct idling.

Raf: Sound logical... cool

So I went out an bought a new fuel cap. No problems so far - see how it goes.

Anyone have any though comments? think what he was saying is crap?

If this is true and it works it can possible save you crap idlers hundreds of $$$$ .. Cap cost me $20.. Well if if turns out good or bad I will let you all know.

[85URK]

im going to keep an eye on this thread, plz let us know whether you have any stalling after a week or so ok. thanx.

[rmahnovetsky]

will do

[rmahnovetsky]

Filled up first time with $20. I noticed when I let the cap off less pressure escaped Good sign so far.... Filled her up, paid for it, and gave her a start. Woohoo idle was fine not a problem there and then.. But driving down the road letting the gas off I still got the backfire - not as bad though. Then on the odd case when I let the car warm down for ~20 mins and give her a start the idle would be hunting again .

I need to fill up again today, so I wil check if the idle problem after filling has vanished.

I also notice that my fuel pressure drops to 10 from 34 psi within 2 hours or so. I was told by the same mechanic that my one way valve in the pump could be stuffed - this could also cause problems. I should check this..

[rmahnovetsky]

I have now filled up three times since my new fuel cap.. So this is what I have noticed so far.

First time fill with new cap (optimax): Idle was fine bit of back fire when off idle.

Second time(optimax): did not notice any difference. Idle was perfect and no backfire

Third time(optimax): Again idle spot on and no back fire.

mmmmm why did I have backfire the first time. My guess is that the fuel I had in the tank before was of a lower octance, it was premium star fuel I think. So my computer would have been running richer as soon as I put the higher octane in and thus causing the backfire..

As I fill up for the next month I will add more notes on my success or failure with the fuel cap.

[rmahnovetsky]

Added more fuel once again. Once again no idle problem Im now convinced the fuel cap has fixed my idle problem after adding fuel...

[The Dan]

It is very possible that the vacuum from your engine was drawing fuel from the tank and up to the purge canister. This would have been caused by the lack of pressure hold in your fuel tank. Good work for solving your problem. I apologise for not getting back to you sooner as I saw you were getting on top of it and I got very tied up with my turbo conversion.

[rmahnovetsky]

no probs 3LIT3 FORS3.

So far: fuel cap definitely worked for the after fill up situation. But I still get idle problems after a cool down of 20 mins or so. Still investing this problem.

[benl1981]

Record thread revival?

I have a similar issue with backfire on decel that is worse after fill up.

I see a new fuel cap helped this in this thread. I assume that the cap has been replaced with a vented instead of non vented?

Stock should be non vented with the charcoal canister used to release excess pressure and recirculate fuel vapours.

I think at some point the charcoal canister on mine has been replaced and only one line to it. Maybe it's not working properly?

Is best thing to do drive it without fuel cap and see if that fixes issue.

If it does fix it the

1. either reinstate all the fuel vapor lines. Or

2. block it off and use a vented fuel cap

Any ideas?

Thanks

[KyuubiGTR]

i wish i found this before i spent heaps of coin on dyno checks and so on...good find thanks for sharing

[bluey33]

Ben just make sure your fuel breather to charcoal canister is fine. Then vent charcoal canister to atmosphere. (block off vacuum lines at engine of course)

See if that fixes it temporarily.

That's what I did as I was interstate when I had issues. Had plans to fix it but still venting to atmo.