Audi 100 backfires on deceleration

I

imported_avcraven

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My Audi 100 backfires quite badly when decelerating, also it only does about 20 miles to the gallon and performance is down. Spark plugs look a normal colour and emission gases are within spec.

Anyone any ideas. garage have not been able to find out the problem.
 
Sounds Like MAF Failure....(Mass Air Flow meter) hence the back firing and lack of power. Could also be Lambda Probe Failure...kinda same symptoms...try replacing them.... /ubbthreads/images/graemlins/thumbsdown.gif
 
Add to 28v6 items, 1. a incorrectly fitted timing belt, 2. a possible vaccum leak, 3. blocked injector, 4. blocked exhaust.

if you work through these items you should be able to narrow down to the fault.
 
Thanks to all those that have given suggestions. I will now tell you what I have done.
Replaced the following:

New Mass Air Flow Meter
New Lambda Probe
New Knock Sensor

Timing has been checked and is OK

Have tried a couple of bottles of fuel additive to clear any blockages in the fuel injectors.

Don't think the exhaust has a blockage as there seems to be quite a force of gases coming out.

The one thing I did notice when this fault first appeared was a change in the exhaust note, it sounded much deeper and boomier. No one seems to think this has any significance though. There are no holes in the exhaust.

Other than replacing the injectors or the ECU I can't think of anything else. This is an old car (163,000 miles) and I can't really throw any more money at it.
 
If the exhaust has changed, then try taking it to an exhaust specialist. It may be that one of the baffles inside has come lose and has caused a blockage or partial blockage.

That would explain the deeper noise you noticed.
 
OK audi5e, that's probably the cheaper option, but I do find it hard to believe there is a blockage as the exhaust gases really do come out with some force, and I thought if a baffle was loose I might hear it rattling about sometimes. It may well be something simple like that but how would it cause backfiring, to me, fuel must be getting into the cat and igniting there.

Thanks for your input i probably will get the exhaust looked at.
 
It could also be a choked Cat, this would also cause the gases to come out with force, also deaden the power of the car, more importantly change the note of the exhaust that you noticed.
 
is there any way of checking for a choked cat. Quite a lot of money to change a cat and then find out it wasn't that.

My emissions are well within spec, wouldn't a choked cat make the emissions bad
 
What about Sourcing a secondhand ECU to TRY only and see if it improves or not..just a thought...
 
A few more detailed observations I have made are, when I start the car from cold and drive it, it doesn't seem too bad, nearly acceptable. Still backfires, but not quite so much. When the car warms up the performance drops off a bit, it backfires more, even backfires when changing gear.

Starting the car from cold and holding my hand near the exhaust there is a very large force of air coming out, blows your hand back abit, when the car has been running for a couple of minutes the air from the exahust has considerably less force.

The fact that the car appears worse when the car is warm should point to a heat related sensor but I think I have changed most of these, think the water temp sensor is Ok as the car runs at normal operating temp.

I know the cat has to get to temp before it works properly but it doesn't send any signals to the ECU does it.

Any thing I replace now is going to cost, so want to try and definately pin point the problem.

is any of this detail any help
 
audi upgrades
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Where exactly are you holding your hand, tailpipe or under the manifold in the engine bay?

You are correct about the CAT, the only signal that should be sent from it is via the Lambda Probe (New) so therefore no problems there ...

Can you photograph your engine so that we may see around abit, just looking for clues and trying to help...

I have an idea, but I'm not sure yet about its validity, Hence a looksee would be nice...

HTH
 
Right heres my Two pence of info

After reading this it looks like you have checked everything possible except mechanical,when decelerating your engine will produce the highest level of vacuum inside,the problem that you have is that fuel is getting burn/ignited in the exhaust manifold/pipe ,what can cause this to happen the timing could be out which in turn would cause the plugs to be sparking when the exhaust valve is open but if this was the case it would be happing all the time so thats not the problem but unburnt fuel is getting to the exhaust so if i were you i would do a compression test on the engine and see if there is any noticable differences between the cylinders as it looks to me the valves seats arent sealing so it could be worn valve seats or the clearance on a exhaust valves from the cam could be to short or knackered valve spring (163,000 on the clock)

This would give you lack of power and higher fuel consumption and backfiring

My Thoughts

...................Steve
 
Thanks for that Rev-head, I had a new cam belt fitted about 6 months ago, and the car was running ok after, so don't think the timing is out, is it possible for the timing belt to slip.
Will investigate into a compression check though.
 
As its a belt, yes - it is possible to jump a tooth, and give bad timing, its worth checking this again, to be sure, don't always take for granted that because something was replaced that its correct (No Offence intended) had this happen to me by a dealer of all people, result : wrecked an engine coutesy of VAG!!! The usual mistake on a timing belt is to NOT tension it before applying the tensioner, OR to allow the crank to move 180degrees putting the timing out to the opposite side......as rev says worth checking again before any damage occurs, i.e bent valve train!!!! /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/thumbsdown.gif /ubbthreads/images/graemlins/confused.gif
 
Have had another diagnostic check today, the timing was at 5 degrees, should be 6 so not enough out to cause any running problems. No actual equipment faults found as such, although the guy said there were some readings that were not right, not having seen the software I can't comment exactly on what he was on about. Where the software was monitoring various parameters, one of the groups was reading 000 and another was fluctuating wildly. He said this was not right but didn't know what it meant and would have to phone audi.

He said it was pointing to a ECU fault.

Will have to wait for his findings and possibly source a secondhand ECU.

Will let you know the outcome.
 
Forgot to mention compression test OK, guy said my engine was in superb condition for the mileage, except for my present fault.
 
out of interest, have you changed the throttle position sensor? (not sure if your model has one?)
It should sit somewhere on the throttle body. If that gets dirt inside or water then the ECU will get strange readings and may send too much or too little fuel into the combustion chambers. This sort of thing can be very inconsistent. It may also corroborate the wild readings??? (again the guy did not say what those readings were for...)
 
another potential could be an intermittent connection in the wiring for one of the sensors.
I had a problem with lack of power (though no backfiring), part of the problem turned out to be the temperature sensor wiring plug! The plug is in two parts, an inner and an outer. if the inner part is not pushed all the way into the outer section, then the wiring does not make consistent contact with the sensor. Hence the ECU was getting intermittent signals from the engine coolant. This problem may happen with any number of the switches on the car. (the other part of the problem was the throttle position sensor/potentiometer, this was full of dirt and moisture, the car ran perfectly when cold but the moment it got above a certain temp the "paste" would become conductive and cause all kinds of chaotic signals to the ecu)
 
OK audi5e, will have a look at the throttle position sensor, pretty sure the temp sensor wiring is OK.

Cheers
 
Doesn't look like this engine has a throttle position sensor, if there is I can't see it and it is very sketchy in the haynes manual.
 
After re-reading all the posts above, I still suspect the exhaust. Call it a gut feel. Just the fact that you mentioned that the exhaust note changed. I would use that as your clue (in hard problems like this always ask, what has changed...)
 
Hey Glad!

The link to the piccies don't work anymore, any reason why?? was looking to see if Audi5e was correct abot the throttle sensor, should be sited on or around the throttle body, can you help on the photos again???
 
Right from the start I have suspected the exhaust, but my I keep telling myself its just a metal tube with baffles how could it cause this problem. I might have to bite the bullet and change it, but I am on a limited budget so can't do anyhting for the moment.

If it was the exhaust, surely it would not be temperature related?

Sorry about the pics not available, due to the server going down. They will be available around another 4 hours, although you should be able to acces them now by typing -
http://64.191.73.117/~chrisjp/audi/
 
Just had a quote from ATS for a new exhaust - the first box behind the cat is £144.48 and the rear box is £208.21, a lot of dosh.

They said they cannot get hold of a cat, are they a dealer only item.
 
audi upgrades
was just looking at the pics, does your airbox reroute incoming air depending on temp from either the exhaust manifold or from outside. If the temp sensor in that box is faulty it may close the air intake or partially close it or perhaps close both inlets?? (probably just clutching here)
 
[ QUOTE ]
Just had a quote from ATS for a new exhaust - the first box behind the cat is £144.48 and the rear box is £208.21, a lot of dosh.

They said they cannot get hold of a cat, are they a dealer only item.

[/ QUOTE ]


If you go on this Web-site you can get the whole standard system for £150.00

http://www.cats-direct-shop.co.uk/exhaust-systems.php?recordID=61

/ubbthreads/images/graemlins/beerchug.gif
 
Sorry just noticed you cant get CAT from them

******!!!!!!!

Rest is cheap i suppose
 
Thanks for the links to the exhaust places, I'll look into that.

Regarding the air intake, I believe it does either let warm or cold air in and I have seen the sensor controlling the flaps. Can I run the car without the air cleaner and lid to see what the flaps are doing.
 
Just check them when the engine is cold and again when hot, rather do this than running the engine without them the filter in place. Keep the ignition on but engine off, then any controllers should operate if they are electrically operated.

I still reckon your exhaust is the cause though, this was just something worth checking first.
I agree with rev-head, rather wait a little and get the stainless steel exhaust system once finances allow. It will save you money in the long run. I have had to put on mild steel exhausts in the past and they only lasted a year (due to all the water produced during combustion).
 
Have checked the flap when hot and when cold and it flips between the two positions. There is no way that it could block off the hot and cold air inlets.

Have phoned the garage guy today about the erroneous readings I had on the diagnostics, but he has still not heard back from audi.

Could it be the air temp sensor which is mounted on the side of the air filter box? Think I shall wait to hear back from audi.

Thanks once again for your input.
 
Finally took my car to another garage as I wasn't getting any results from the last one, think he had given up. The next garage went back to basics and checked the timing first, this was found to be 7 deg out, why the first guy didn't find this I don't know. Anyway it couldn't be adjusted there and then as the distributor was seized. It has now been put right and performance is back, I have yet to run it some miles to find out fuel consumption.

How the timing got out in the first place is a mystery. Thanks for everyones who contributed with advice.
 
The first garage said he did it as part of his diagnostic check from his laptop. The guy in the second garage said he always checks the timing with a strobe and finds that more accurate.

I must admit it's worked wonders for the performance and it really pulls up hills now. Has also cured the backfiring on de-celeration.
 
Forgot to mention, the sound from the exhaust is much quieter now. This could be a good clue for anyone else that develops a similar problem.
 
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