Backfire

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Backfire

Postby Danevansgolf » Mon 19.06.2017, 09:30

I've just fitted the pnm standard full exhaust system (no silencer fitted). Been out for a drive and noticed that the car the car backfires a lot on overrun (when I take my door off the throttle). And also noticed my fuel consumption has seriously decreased. Also when driving the warning light comes on for about 2 seconds and goes off again, does this about 3-4 times during a 35 mile trip! Any ideas????
Obviously it sounds like it's running very rich. How do I go about either changing it or adjusting this problem???
All ideas welcome please.
Slightly bit embarrassing driving through a town, with my car wanting to backfire. Cheers all.
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Re: Backfire

Postby Rambo » Mon 19.06.2017, 10:14

The popping and backfire on overrun or changing down is fairly normal if you have a PNM or Piper exhaust. It is just unburnt fuel combusting.

Are you running with a Everest chip ? If so, you might need your wastegate porting. See this topic.......

http://wikilec.com/view/Porting_the_was ... _wastegate

Engine light coming on isn't normal. Have a quick check with your paperclip and see if there are any stored error codes.
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Re: Backfire

Postby Danevansgolf » Mon 19.06.2017, 12:18

Had a mountain chip in for a few years now. I get a lot of popping then a few loaded bangs, obviously it sounds like I burnt fuel. Had thought about waste gate porting. Will get the paper lip out later and see. Doubt anything will show as is come on then goes off pretty quick. Will let you know on that result.
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Re: Backfire

Postby HJ2 » Mon 19.06.2017, 16:47

My bet is on porting as well. :poke:
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Re: Backfire

Postby Artaban » Mon 19.06.2017, 17:09

Its normal for the engine light to come on for a few seconds after heavy acceleration as fuel cut off kicks in. I can only get mine to do it in 3rd gear. I have the PNM exhaust and haven't yet ported the waste gate. I understand porting will solve all my problems.
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Re: Backfire

Postby Danevansgolf » Mon 19.06.2017, 20:47

So. I've read the Wikilec on porting the wastegate. I understood about the first 2 words and got lost after that. Do I need to take the turbo out all the way to the exhaust down pipe??? Is it the point of just undoing everything and taking the whole of the turbo out?? Answers on the back of a postcard please. Ha ha Cheers all.
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Re: Backfire

Postby rip » Tue 20.06.2017, 07:35

You will indeed need to remove the turbo to port the wastegate, but why would this stop overrun?
The wastegate controls the turbo. When you close the throttle (which is when you get your overrun), the wastegate will close so it is therefore unrelated to your overrun issue.
Mine suffered for excessive overrun a while back. It sounded really nice but boost gave me little performance & I hit overboost very easily.
I experimented with ignition timing & found that mine needed to be set way in advance of what the timing mark says. The performance difference was huge & the turbo now gives me a big kick in the back.
I am convinced something under my bonnet is out of alignment, giving me a false timing reading.

My advice is to experiment with your timing. If you mark its position before you start, then you can always replace it to where you started if you don't get good results.
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Re: Backfire

Postby Jamie N » Tue 20.06.2017, 09:40

Danevansgolf wrote:I've just fitted the pnm standard full exhaust system (no silencer fitted). Been out for a drive and noticed that the car the car backfires a lot on overrun (when I take my door off the throttle).


Maybe that's the main problem Dan?, something that heavy on the throttle isn't going to lift too quick, you should try the shoe idea, some say its a superior system :P .
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Re: Backfire

Postby HJ2 » Tue 20.06.2017, 10:54

:lol: :lol: :lol:
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Re: Backfire

Postby Rambo » Tue 20.06.2017, 16:29

Jamie N wrote:
Danevansgolf wrote:I've just fitted the pnm standard full exhaust system (no silencer fitted). Been out for a drive and noticed that the car the car backfires a lot on overrun (when I take my door off the throttle).


Maybe that's the main problem Dan?, something that heavy on the throttle isn't going to lift too quick, you should try the shoe idea, some say its a superior system :P .


Barefoot is even better than doors on your feet :twisted:
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Re: Backfire

Postby Danevansgolf » Tue 20.06.2017, 17:04

Cheers for the comments lads. You realise you all own a sports car!! They are meant to be driven with fun, excitement, and the wind blowing through what hair we have left on our heads. My foot is part of my heart which in turn is part of the lotus, which also loves the speed!!
I will try adjusting the timing slightly and see what results I get.
But I thought if everyone experienced excessive back fire i thought it was the wastegate that needed porting. Will let you guys and girls (the ones who wears slippers when driving) know what's I find. :D :D
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Re: Backfire

Postby Giniw » Tue 20.06.2017, 18:31

Danevansgolf wrote:Doubt anything will show as is come on then goes off pretty quick. Will let you know on that result.
The errors codes are stored so you will probably be able to see the error code it has detected.

As for blindly adjusting the timing, I would be cautious about that.
Actually if you suspect your flywheel mark is out of alignment, it's easy to roughly check it by removing a spark plus on the first cylinder and turn the crankshaft until the cylinder is at the top dead centre position. If the timing mark on the flywheel is next to 0°, then the marking is all right and the problem is probably elsewhere.
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Re: Backfire

Postby rip » Wed 21.06.2017, 12:01

Giniw wrote:As for blindly adjusting the timing, I would be cautious about that.

That is why I said to mark it before starting. If anything goes wrong, you can always reset it.
How do you think engineers set it in the first place? If you design a brand new engine, you need to test to find the optimum spark timing. You don't just set it to the value used for the previous engine & assume it is still correct.
Many of us have fitted bigger exhausts to our cars. I do not assume that this has not affected the dynamics of the engine If you make a change somewhere with any engine, it is possible that you have to adjust something else to get the best from it.
With any engine if you change the cam profile, exhaust, port the head for better flow then, assuming you have an electronic management system, the last thing you should do is re-map. A proper custom job will include a rolling road test in order to provide the optimum fuelling & ignition for the changes you have made.

The pulley only goes on 1 way because it has a woodruff key so I have no idea what could be out with mine. It could even be he mark itself, which is such a small nick in the wheel that it can easily be mis-read.

The difference when I did my timing by stopwatch was quite alarming.
My test was to accelerate from 25 to 70 in 3rd along a flat, quiet dual carriageway, starting the watch when the speedo hit 30. Adjust the timing then try again. Adjustment almost halved the 30-70 time.
I also hooked up Elanscan & watched for knock to make sure the ECU was not backing the timing off. When I saw this happen, I knew I had gone too far (& I got no more performance at this point either).
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Re: Backfire

Postby Giniw » Wed 21.06.2017, 19:33

rip wrote:How do you think engineers set it in the first place?
I don't know.
I would have said that it's being estimated with both the pistons speed (engine revs) and the combustion speed constant(?) and the engine tendency to knock for a given petrol octane value? But I reckon it's probably very simplified, if not all wrong ^^

Many of us have fitted bigger exhausts to our cars. I do not assume that this has not affected the dynamics of the engine If you make a change somewhere with any engine, it is possible that you have to adjust something else to get the best from it.
With any engine if you change the cam profile, exhaust, port the head for better flow then, assuming you have an electronic management system, the last thing you should do is re-map. A proper custom job will include a rolling road test in order to provide the optimum fuelling & ignition for the changes you have made.
I agree with that, it's just that offsetting the base timing is not equivalent to remap all the ECU tables, but I am not a specialist so maybe it could be OK. I just don't know.

The pulley only goes on 1 way because it has a woodruff key
Yes that is why I find it strange and suspicious to alter the timing without investigating more. But that is just my not so informed understanding, it's just that I would be afraid to do that on my car. I don't say it is actually a wrong idea ^^

The difference when I did my timing by stopwatch was quite alarming.
My test was to accelerate from 25 to 70 in 3rd along a flat, quiet dual carriageway, starting the watch when the speedo hit 30. Adjust the timing then try again. Adjustment almost halved the 30-70 time.
I also hooked up Elanscan & watched for knock to make sure the ECU was not backing the timing off. When I saw this happen, I knew I had gone too far (& I got no more performance at this point either).
Well, indeed if you can see no knocking, it's probably not bad :)
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Re: Backfire

Postby Brit-Car-Nut » Wed 21.06.2017, 20:36

Someone apparently read Mr. Smith's (Phillip H.) book on tuning for speed...

Back in the carburetor days (especially SU/Strombergs which need engine vacuum to work correctly), road tuning was a good way to squeeze a little bit more performance out of a tune-up.

BUT...

Modern cars with ECU logic and a lot of very fast sampling of the state of running, cranking the timing up to get more performance might really be over-riding something (other than knock) that the ECU knows but you don't see.

Setting the timing a little (2 degrees or so) more advanced might give a little oomph, but more than that means something is WRONG.

First things first: are the timing marks accurate? Is/Are the cam(s) in proper time? I have seen way too many belt driven cams that are 1/2 tooth out and cranking the timing will make it better, but the engine is still not running correctly.

How do you get 1/2 tooth out?

When changing the timing belt, you set the cams in tune (mark to mark or whatever the process), and then rolling the belt over the pulleys and if everything lines up, it is in time, eh? NOT ALWAYS.

What most people don't realize is there is a fair amount of slack in the timing belt and it is really easy to end up with the slack AFTER the tensioner and after the crank pulley which after around 20 engine rotations will put your cams slightly out of time. My neighbor recently changed his timing belt and just lined everything up and didn't realize the slack was not at the tensioner. He tightened everything up and drove to work only to visit a few days later to complain that something was wrong with his car since he had to advance the ignition timing to get it to run ok-ish. After removing the belt cover and trying to line up the marks, the cam that drove the distributor was off by enough to retard the timing.

LESSON: When changing cam belts, make sure the "natural" slack is AT the tensioner and the timing points line up. On distributor-less ignitions, it is harder to figure out poor running, but I always make it a habit to get everything set up as I interpret the manual and then before closing everything up, I crank the engine at least 20 times and recheck that the timing marks still line up.

When I did my timing belt on my M100 (20,000 mile car but in 2009), it looked fine at first but after the extra rotations, I realized I had to re-set the alignment. A lot of people get it right the first time - some through experience and others by pure luck. Anytime you have to advance the ignition timing beyond the normal setting (allowing for a slight increase for that "little bit more power"), then there is something wrong that can be a stretched timing belt, damaged keyway(s) at the timing gears/sprockets or some other mechanical issue that should be resolved so the engine doesn't destroy itself trying to run out of time.

Rip: I don't have any idea why you had to tweak the timing to get a better run, but after 55 years of engine tuning, something sounds (to me) wrong.

The original post was asking why AFTER changing to a more free-flow echaust, the car was now popping and backfiring. Those symptoms are more an indication of overfueling than timing and unless he is trying to run the car at very high boost, the wastegate isn't really an issue. Cams out of time will cause it, excessive timing advace will cause it but so will excess fuel that then gets burned/blown up in the exhaust.

I would be inclined to check the air/fuel mixture, fuel pressure regulator and engine timing including timing belt alignment. That said, "back in the day", a less than perfect exhaust system always popped and farted since back pressure was never correct.

I really don't see how ignition timing can compensate for back firing (unless it is found to be way off to begin with). Remember, to set the timing on the M100, it has to be in test mode - that is the ECU is NOT adjusting the timing to meet the currently sensed situation.

Also, look at the temp sensor which might be sending the wrong engine temperature to the injectin system thus running the engine richer than it needs to be. The temperature gauge on the dash is NOT an indication of what the fuel injection thinks the engine temperature is. Again, an Air/Fuel exhaust pipe test will help to narrow down mixture issues but a bad temp sender will make everyone crazy.

Read the other posts that list the resistance values at different temperatures. Verify that your sender is close to those readings.

No intent to snub or criticize anyone, just my 2P.
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Re: Backfire

Postby Danevansgolf » Mon 26.06.2017, 21:23

We'll have done the engine diagnostic with the good old paper clip, only to find no codes are stored which is a good thing. Then realising that it wasn't the engine light coming on, it's the warning light flashing on then off when deaccellarating/ braking. Not sure why. I have started to take apart all the pipes and accessible bolts from the turbo. No going back now as some head bolts have broken off, so it's a case of forward slow progress only. At least I can drill them out once it's all on the bench. I feel there will be a lot more!! Keep you posted. How on earth do we get to the 3 bolts holding the turbo to the engine off?? I can't even get a spanner in!!!
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Re: Backfire

Postby HJ2 » Mon 26.06.2017, 22:04

It's easiest when you remove Turbo + exhaust manifold in one piece.
Then you can get it of whilst on the bench. Keep an angle grinder ready to make some old open end and ring spanners fit the job :-D
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Re: Backfire

Postby Rambo » Tue 27.06.2017, 07:39

Danevansgolf wrote:Then realising that it wasn't the engine light coming on, it's the warning light flashing on then off when deaccellarating/ braking. Not sure why


Check that your brake fluid reservoir is fully topped up
Last edited by Rambo on Tue 27.06.2017, 07:44, edited 1 time in total.
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Re: Backfire

Postby HJ2 » Tue 27.06.2017, 07:42

:agree:
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Re: Backfire

Postby Simon_P » Tue 27.06.2017, 09:44

When you changed your exhaust did you reset the ecu? If you didn't it will have the old learnt values which will now not be correct.

Power vs ignition timing is like a hill, peak power is at the top. Knock is the limit and is at the bottom of the hill on the other side.
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