Intermittant backfiring 1988 1.9 petrol

[triumph2.5pi]

Hi. I'm wondering if anyone can suggest possible causes for mystery intermittent backfiring? When it happens it can be difficult to start and "hesitates" a lot under acceleration.
It's been happening occasionally, say once every few months for a long time, maybe 3 years. It's been checked out by two T25 specialists who haven't been able to find the issue. Of course when the van is in the garage it runs perfectly so difficult to diagnose the fault!
It has been in for at least 3 full services in these three years where the usual culprits have been replaced more than once ie condenser, cap, leads, plugs etc.The emissions are fine.
The only noticeable running themes of the symptoms were that is seemed to happen at the start of a new journey from cold, when it was very damp or it had been raining very hard. The symptoms never started well into a longer journey. More recently though it's even done it when dry (although less so)
Someone suggested water may be getting in the petrol from the petrol filler pipe (I'm told they often rust there), but that's been checked out and it's all fine.
As the symptoms were so intermittent, I had just put up with it, but the other day it refused to start at all until the battery went flat and it's definitely getting worse The next time you go to start it though, say later in the day, it's perfectly fine again!
It's booked in at a VW specialist again on Tuesday, but they've looked at it before and not located the issue.
So I'm just wondering anyone has any ideas on what it could be that maybe hasn't been looked at yet...? Any input much appreciated , ta:-)

[123-jn]

If it only happens when cold and therefore on choke I would think it is fuel related, I had a golf with the pierburg carb on and it used to be like that which was a real pain as it was automatic and used to stutter backfire and stop when trying to pull away at junctions. I wonder if the choke is working correctly. There are a number of parts on the choke including a bimetal strip which opens the choke and reduces the extra throttle as the coolant heats up and a vacuum operated device called the pull down unit which opens the flap as the vacuum increases.(with the engine running you should be able to push the choke flap to within about 5 mm of shutting manually, if you can't the pull down unit is not well) Check the vacuum pipe that goes from the vacuum stubs at the back to the pull down unit and to the distributor and air filter hose id cheap and it is worth replacing them. I suggest starting the van with the pancake off so you can see the carb, check that the choke is fully on at start up and that the throttle advances onto the top cam. Now keep an eye as it warms up blip the throttle a couple of times as it warms the throttle should drop to the lower cams and slow down. The choke flap should slowly open. Once it begins to warm up the choke flap should not close as you throttle up. If things don''t operate right you can get a rebuild kit on line and instructions as to how to set up the carb after rebuild. Then you will also replace all the gaskets and should have no inlet manifold leaks either. Once it's running get a garage to set the mixture.

[kevtherev]

From what you describe your engine is not running with standard ignition.
This to me is a retrograde step.
I would be refitting the electronic ignition.

[triumph2.5pi]

Thanks so much for the input, much appreciated
Most of it's too technical for me personally to be honest(!) but I'll bring this all up with the garage tomorrow and hopefully find the fault

[triumph2.5pi]

From what you describe your engine is not running with standard ignition.
This to me is a retrograde step.
I would be refitting the electronic ignition.

Thanks Kev. To be honest I don't know what ignition it's running, It certainly hasn't been "upgraded" in my ownership of 4 years or so, but of course could have been before that...Anyway I'm just curious to know why you think it might not be standard?

[ghost123uk]

Because you said the condenser has been changed. VW T25s don't have a condenser in their distributor. Condensers are only for the old fashioned "points" ignition system, not our modern electronic ignition system. I bet you were paraphrasing (or whatever word is appropriate) when you wrote about the condenser being replaced, or were you?

If it has the correct ignition system, than as 123-jn says, it does sound carb related. Between him and Kev I bet it'll get sorted

[CovKid]

Lets have a photo of inside the engine bay then you'll have a team of us looking for you. Theres quite a bit we can tell even form a basic photo. Yegads, there could be a 009 in there.

[123-jn]

Let us know the verdict It's always nice to know if we have missed something or even if we were right!!!

[triumph2.5pi]

Thanks a lot everyone. Yes good idea, I will take a photo tomorrow and post it.

ghost123uk - re condenser etc, yes sorry I am clearly showing my lack of mechanical knowledge lol! I was just trying to underline that this has been a long standing problem and it has been in for numerous full services and also specifically in at T25 specialists to sort this issue to no avail as yet.

The last time it was in, they "looked at fueling" and told me "the emissions are alright". I don't know if they've looked at the choke. Anyway it's in again tomorrow so I'll bring up what has been mentioned here. Thanks again

[kevtherev]

At best remote diagnosis is difficult.
Through a third party with no idea what does what, and what has been done will be at best confusing.
So with respect, you need to be more involved with your engine.
We or you, do not have a clue what has been eliminated from the investigation.
So for me you need to list the components and tasks that have been done.
Simple things like fitting the correct parts is down to you and simple tests we here advise also.
Sorry to be so blunt chap.

[triumph2.5pi]

Hi all
Here are some pics of the engine bay. The garage couldn't diagnose the fault yesterday as it wasn't displaying symptoms so I've left it with them so they can start it from cold this morning. I'll keep you posted!

[Tinium]

Someone suggested water may be getting in the petrol from the petrol filler pipe (I'm told they often rust there), but that's been checked out and it's all fine.

I had the same fault, the filler pipe was fine, it was one of the breather pipes that was shot (in the wheel arch on my Syncro, so right in the firing line of all the spray being kicked up) - might be worth a double check of these, if you've not done so already. It looked OK, but a strong whiff of petrol was giving the game away

I've had occasional struggling under full acceleration - tracked that down the vacuum tube to the distributer being rotten at the dizzy end, cut the last inch off and has been OK since.

HTH
Ben.

[chrisbeck]

Hi
Sounds ignition related to me, could be anything from a hairline crack in the rotor arm or distributor cap, to knakered vac advance on the distributor, dizzy cap and rotor are dead easy to change yourself, and only cost a few quid, and the vac advance is easy to check. Also check all vac hoses, also cheap to replace and can be bought by the meter. you could or should also check the carb to manifold anti vibe gasket under the carb, do this by running the engine and spraying a bit of easy start around it to see if the revs rise or not, or get hold of the top of the carb and give it a small push in each direction, if the base ruuber is u/s it will cause an air leak which will cause bad running.
Hope this helps a bit.
Chris

[kevtherev]

Some observations from your pictures
Crank breather pipe is kinked
Coil looks original
Spark control unit looks original
Vac pipe connectors need to be changed

I see you have an Autosleeper, like me.

[triumph2.5pi]

Thanks again everyone...So the only definite thing I got from taking it to the garage is...it needs a new clutch lol!
They checked over everything the fuel system, carb and choke, vac hoses and ignition system, started from cold and no issues found. They've said the only way forward, as it wasn't displaying any symptoms (of course it behaved impeccably whilst there!), is to start replacing parts randomly to see if anything makes a difference. Yes Kev, you're right the mechanic also mentioned that the spark control unit and coil are original. Are they likely culprits and worth replacing? Distributor cap and rotor have definitely been replaced more than once since this issue started 3 or so years ago. I need to bear cost in mind too as I'm moving house which is draining finances to the max...
As 99.9% of the time it starts on the button and runs like a sowing machine and the fact I don't have money to throw at this issue, I think I'm inclined to continue to put up with it for the time being...and revisit it if it gets worse (ie won't start again) or when I'm more flush!
Yup, it's an Autosleeper, I love it