Air Leak - Backfire/ popping exhaust on decelleration

I thought i would start a dedicated thread to this, it seems to end up being the subject of any other thread i do so this is it.

For those who are unaware of the issue, 1800 type4 with twin solex 36-40

I have a backfire on decelleration when under load ie. slowing using gears as if coming up to a junction for instance.

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This morning i let the van run after i pulled it out of the workshop and i checked for air leaks around the manifold joints on the carbs and heads, by spraying wd40. I was unsure what sort of engine sound change to expect, but i didnt really get anything that related to the wd40, so i am going to rule out the gaskets/manifolds/carbs area as the source.

Today after work, around 3 ish, i am going to remove the tinware on the heat exchangers, and then do as @lost-en-france suggested and block the exhaust with a damp/wet rag and have a look and listen for leaks at the joins from the H/X's to heads, and from the exhaust flanges to H/X's. I dont think there is any other areas where a leak might occur as far as i know.

It is running really strange right now, sounds a bit loud like a harley or something, sounds a but puffy. So i am trying to run it as little as possible.. I am not driving it currently due to fear of causing damage.

The idle is also really off too on startup, like it wants to konk out. Nothing has been adjusted to make this happen.

All nuts joining heads and flanges etc have been checked for tightness. I have a suspicion that there may be a stray copper gasket/washer missing from H/X to head somewhere, courtesy of previous VW 'Specialists' but i have yet to see, it is just a hunch.

As previously mentioned, i have a rolling road sesh booked for the 27th July, and if it is backfiring, i doubt there is much point going.

Will report my findings tonight after i finish.

 

Looking at previous posts you have a brake servo - have a good look at all of the pipework for this from engine bay all the way to servo, and do a servo test to check the diaphragm(engine off, pump brakes a few times until pedal goes 'hard' then hold pedal down and start engine - as it starts pedal should drop as vacuum pressure builds in servo).

Any issues with this pipework and its like having a 1/2 inch hole in the inlet manifold and it'll run mega lean. Can be hidden by braided pipe work that's perished inside as well, so if you find any braided hose pop it off and have a look inside....

good luck with it.

 

get it underneath and take the bottom tins off that cover where the HX fit into the heads. Any leaks are likely to show as sooty marks and be fairly obvious.
as Orangefeeling said above, check for air leaks on the servo take offs and balance pipe although I would expect you to have stalling issues if you had a leak in that circuit

 

Ok so I inspected the heat exchanger to head area. There appears to be leaks both sides. However I can see the copper gaskets are there. One side has a slug weld that isn't being very useful and also has soot around the weld. Didn't need to block the exhaust, I can clearly see there is a seating issue where the two surfaces mate. Here's some pics.







Also there is an oily mess above the lhs push rods picture below. Not sure if related or old. No way to tell. The pushrod tubes leak a bit but not upwards I'm sure.





So obviously I need to tackle the hx to head area as one item on the list.

What's your opinions? (Experienced members only if poss - not being rude at all, feel free to spectate unless you have concrete knowledge)

Cheers all.

Psg

 

Heat exchanger is broken.
And you shouldn't see so much of the copper gasket.
And you have an oil leak.

 

Ok. So what would you do next? Take hx off and see how to fix?

And oil leak isn't leaving a concerning amount of oil on the floor. Just a tiny drop probably once or twice a week about 10p size.

The copper gaskets move as you put them in lol. I had them on right when I did it I think but like I said they have been taken of by a couple of doosh bags since then.

 

Copper gaskets shouldn't move, they sit in a seat. Were they anealed behore you fitted them? Ratwell has a good article on filing the hx mating surface flat with a long file to make sure they seal properly. And use K-nuts (the one on the right in photo) on the manifold to make sure they don't come undone or rust.
That's all assuming you hx isn't broken, but do it anyway with a new one.

 

I don't remember them sitting in any kind of seat. The end of my hx pipes that go to the heads were flat iirc. when I fitted
Them I had to balance them on the ends whilst trying to fit them. Took me a few goes.

 

Really? I had to pry mine out with a spikey screw driver. Are sure you didn't put them on top of the old ones?

 

Haha no idea. I was advised to get some. The hx's have got to come off I think anyway so I will inevitably find out soon.

 

I always use exhaust paste on everything ,would'nt trust the copper....

 

Which one

http://www.eurocarparts.com/car-exhaust-repair

 

http://www.eurocarparts.com/ecp/p/c...lants/car-exhaust-repair/?549771941&0&cc5_138

the top one is big holes...

 

This! There is a gasket shaped hole up there.
And the HX is broken. The weld someone tried already to mend it with is cracked, you can see the gap. It doesn't weld very successfully, it's cast.

 

New heads have the seat machined too wide. The copper washer moves about unless you stick it down with something first.

Sent from my SM-G920F using Tapatalk

 

Take the heat exchangers off, inspect them, if all is well check the copper sealing rings if ok you can re use them if in doubt buy new, smear with a bit of silicone and push ring into place with a flat head screw driver, there is a slight ridge that will hold the ring in place, check all the studs, renew any that are Marmite, finally bolt up H/E in sequence ie first nut by hand then the next etc, then do the same with a socket, don't be tempted to tighten each nut fully or you could snap the lugs on the HE and will have to get it welded, ask me how I know that, good luck

 

If you have an extra gasket in one port, but not the other, that might explain the weld cracking, but would certainly cause a big leaks or two.

 

Cast can be welded, but it's difficult, the cast iron needs to be pre heated and nickel rods used, also you can't weld two bits together but it has to be built up, best give it to a specialist welder, I found the place that did mine with a simple google search

 

because it is welded onto the thick piece it should weld ok ,if it was just the pipe , even I would chuck it...

 

This could be a really stupid question but have you fitted early exchangers onto late heads. Cant quite make the pics out but the copper rings dont look right. Btw i dont actually know if this is possible! What year engine is it?