Backfire

[jgbff7]

I would be grateful for advice pointing me in the right direction, please.

I like my S2 M100 so much I have bought another as a bit of a project. SE, and 58000 miles, 1991. I have changed the cambelt, changed the cooling pump and painted all of the suspension.

It idles beautifully from cold but on the road it backfires and then continues to backfire at idle.

I did the paperclip and test and get code 31, Turbo overboost.

The car has an induction kit on it. It looks like a dump valve and there is another smaller silver thing down near the turbo which is embossed "Turbo." It also has a KN airfilter.

It has not been used much in recent years and the petrol may well be stale.

I would be grateful for any thoughts please.

John

[dapinky]

Without seeing the bits you mention, I can only guess as to the cause and symptoms....

..... Certainly, if the car is backfiring, then there is unburnt fuel getting into the hot exhaust and igniting....

.... the trick is to guess how and why that is happening.

My best guess is that (as the CEL indicates an 'overboost' situation), is that "something" is causing excess fuel to be injected which is not being burnt..... As the car runs on a 'wasted spark' system with a double-duty injector cycle, it may be caused deliberately or accidentally (due to a fault elsewhere in the system). *if* the part you talk about ("small silver thing") is a manual bleed valve to increase the turbo boost, then that may be part of the problem - but not all of it.

It may be something as simple as a sticky wastegate, or it may be burned-out valves in the head - impossible to say for certain with limited information.

If it were mine, I'd do a compression test to confirm or eliminate any valve issues, and then take it from there, but I cant help further without more clues.

[Tuga2112]

you probably wont go wrong with a new set of spark plugs, specially if you are unsure of how old they are. its a cheap part. although admitedly from my experience, one that certainly lasts 4-5 times longer than the usual maintenance schedule suggests you replacee.

[jgbff7]

I have just done a compression test. 150 across the board.

What was interesting was that last week when I took out the plugs to turn the engine when I did the cambelt, the plugs were exactly as they should be, light brown and clean. Today after only going round the block and getting the misfire they are very sooty.

I shall try and attach a photo but I have had trouble with downsizing before, so bear with me.

[jgbff7]

The silver bit is at the bottom

[jgbff7]

Here is the silver bit

[jgbff7]

One additional thought.

I replaced the cambelt exactly where the old one was placed with all the marks lined up.

If the belt is say one tooth out will the engine run but then run badly? The top wheels were locked but the crank pulley can move although I am sure it did not.

John

[Tuga2112]

One additional thought.

I replaced the cambelt exactly where the old one was placed with all the marks lined up.

If the belt is say one tooth out will the engine run but then run badly? The top wheels were locked but the crank pulley can move although I am sure it did not.

John

if your cambelt is a couple of tooth out it will run like a tractor.
i know it from experience

edit
i just saw the video i put was from a different issue. turns out the one from the time i did the head gasket is not on youtube.

[dapinky]

If the cambelt was replaced in exactly the same orientation as the old one, I would expect it to be fine (okay, the tension may be not-quite-perfect, but no biggie in this case).

Did it run okay before the belt change, or did you do the belt before trying it (which is the safest way to do it!)?

Anyway, if the car only changes it's behaviour when it is hot, it is unlikely to be anything to do with the cambelt.

The compression readings are lower than I have seen on my car, but as my gauge is old and made by me whilst still a student, probably not accurate! - that said, the number itself is far less important than the comparison between cylinders - and if all 4 are within a small percentage, then they are probably fine. I am assuming that the car has been sitting idle for a while before you got it (I know that feeling!!) and as such, the piston rings will have set slightly in their grooves, and the valve seats will have a less than perfect seal on all 16.... so a slightly low reading is expected.... but they are all the same, so I would discount it being an 'internal' issue (at this stage).

If the plugs were fine before doing the belt, then it is safe to say the car was running correctly at the time it was layed up - but during that time a fault has developed.

I would still say that fueling is the issue (sooty plugs support the idea) which is causing the backfire. It is possible that the timing is out (but if you haven't touched the CAS it should be as it was before) and unburnt mixture is getting into the exhaust (I've had that happen before now).

Maybe one (or more) of the injectors are slightly seized and delivering too much fuel? (Fuel pressure regulator fault can give the same result)....

....The silver thing is (as guessed) some sort of MBC (Manual Boost Controller) which basically bleeds-off some of the vacuum going to the wastegate, so the wastegate doesn't open when it should, and thus the boost level builds beyond where the ECU thinks it is...... Although it was the "go-to" performance upgrade back in the early 90's it has problems - and can cause disaster!

The biggest issue is that if it is set too high, the boost will go above 0.92 bar, and that is the point where the standard injectors simply cannot flow any more fuel, and the air/fuel ratio goes beyond 14.7:1, and the engine runs too lean. This can cause a sudden spike in internal temperature and cause the pistons to melt - it won't give you any warning, and will happen instantly, and the first you know about it is a big bang!
To avoid that from happening, as well as the MBC you *should* also have a Fuel Cut Defender, which is a box of (very basic) electrics which causes the ignition to stop working if it detects the conditions where the fuel ratio is too lean...... it is usually a smallish black plastic box with 4 wires, and will be spliced into the wiring on the bulkhead at the MAP sensor (I can't see one in your photo, but it could be just out of shot, or hidden behind the inlet manifold).

Again, I don't see any of that being your actual problem at the moment, but would explain the CEL indication.

I really think it is something fairly basic - maybe down to old fuel gumming up the injectors or similar.

I think Joao's idea of a new set of plugs is worth a shout, and maybe a bottle of injector cleaner in the fuel tank (neither of which will do any harm anyway).

Maybe it just needs a bit of exercise.

[jgbff7]

Thank you very much for such a comprehensive reply.

Yes, the car had been standing, and no, I did not run it as I was very worried about the age of the belt.

I shall try fresh fuel and a dose of injector cleaner.

I did not know about the issues with the induction package. My preferred route is to take it off. I find the performance of my (standard) S2 perfectly adequate. Very interesting that you say this was a mod in the 90s. According to the paperwork that is exactly when it was done.

Should I put back the original air cleaner?

Again, very many thanks.

John

[dapinky]

I did not know about the issues with the induction package. My preferred route is to take it off. I find the performance of my (standard) S2 perfectly adequate. Very interesting that you say this was a mod in the 90s. According to the paperwork that is exactly when it was done.

Should I put back the original air cleaner?

Lotus actually approved the fitting of the MBC kit by their dealers when the cars were new - but only for a limited selection of kits - and your's looks a bit different to the ones I have seen before. It is important to remember that this was still very much the 'early days' of tuning by electronics, as until this sort of time it was all points, plugs, distributers, carburettors and camshafts - anything that was needed to go faster was purely mechanical swaps or machining work...... the chance to just change the programming was still almost fantasy to most owners.

From 1990 onwards it was an exponential curve and such "performance modifications" became old fashioned very quickly with the advent of chip-controlled advance curves/fueling. Certainly, by the time the S2 came out, Lotus were no longer offering such a service, although they were still fitted by some people......

..... either way, it works very well at what it does - actually too well, and without the FCD it is a time bomb.

Removal may or may not be simple - removing (if fitted) the FCT is a simple case of removal and re-route the wiring directly as it used to be (should be plug-and-play). Removal of the MBC is a simple case of taking it away and replacing (or joining) the vacuum pipe - BUT - there may or may not be changes to the OE vacuum pipework concerning the Boost Control Valve (fitted just to the rear and below the OE air filter 'dustbin') - Section 2 of the Engine Management Manual (EMK2), Pages 18/19 show the location and explanation of how it functions.


If you have the OE air filter housing and pipework I would re-fit it - the 'high-flow' filters don't actually do a lot on these cars as there is more than enough airflow to boost to over 1.2 bar (with suitable injectors!) with the standard kit. In fact, they can have a detrimental affect depending on exactly how it sits in the airflow - the OE system has a pipe from the front of the car which basically 'rams' cold air into the system as the car moves forward, whilst the aftermarket system just draws air from where it is.... and that can often be warm/hot air from the engine bay.

The fortunate position you are in is that you have another car to compare everything with, so finding things which have been changed is a bit easier than trying to follow a set of diagrams.