Intermittent misfire proving impossible to diagnose

[Hapless Biker]

Hi there

I?m new to the forum and am actually writing on behalf of my dad, whose Elan has had an intermittent problem for about two years now. We?ve tried everything we can think of and I?m hoping someone on here might be a bit more knowledgeable than either of us!

The car is a 1970 Elan S4 SE and the problem is an intermittent misfire that seems to kick in almost exactly after 10 miles from cold (odd but true). The misfire begins under acceleration.

The car starts fine every time and runs perfectly until the 10 mile limiter kicks in. Even when the misfire is occurring the car will tick over, but just won?t pull under throttle. Once the car is properly cold it runs fine again for, you guessed it, about 10 miles.

The car has been looked at by Aldons and ran fine for about 12 months after they played with it, only for the problem to re-occur.

So far we?ve: changed the coil; new jets on the Webers; Aldons electric ignition (but same problem on points); new distributor and leads; fuel pump re-built; and new fuel tank (so tank and fuel lines clear).

We?re left scratching our heads so any advice would be very much appreciated!

Cheers

Kevin

[AHM]

Mechanical fuel pump? Bowl full of crud.
Webers - correct float levels?

[archigator]

Have you recently installed an in-line fuel filter? I installed one, for the first time, in the line coming out of the gas tank a while back, and had a similar issue. I have a mechanical fuel pump, and apparently it didn't have the "umphf" to draw the fuel at a proper rate through this new filter. The car would run on the residual fuel in the line, fuel bowl, and the carbs... and then start misfiring and run terribly once it had to suck that fuel through that extra filter... after about 5-10 miles. I removed the filter altogether and VOILA!

Just a thought...

Gary
'71 Sprint FHC
Miami, Florida

[RotoFlexible]

Hello, Kevin, and welcome.

Several past posters have experienced similar problems and traced it to coil overheating or failure. Here is a typical example. My own persistent and mysterious misfires have been due to little blobs of silicone sealer in the fuel tank, but the symptoms are different and besides, you have a new tank. Hope you track down the cause quickly.

[AHM]

Good point Gary,

I experienced the same thing when I rebuilt the S3. I put the filter in having had problems with a pump silting up on another car.

It gradually got worse. After the car refused to run having died mid journey, I spoke to my local parts guy about electrical problems.He insisted I change the (new) fuel filter - it worked , and again 2 weeks later.

[512BB]

You will get the symptoms you descibe if the condenser in a points dissy is duff, sorry Mike, but not sure if you have one on your system. If you do, change it.

Leslie

[hatman]

Kevin, if the car's still got the anti-theft switch in the glovebox get rid of it, or at least override it.

[rgh0]

when it starts misfiring does the tacho die in tune the misfire. If it does then its probably electrical. If the tacho is not upset in a major way by the misfire then its probably fuel - that at least gives a starting point for investigations.

cheers
Rohan

[Hapless Biker]

Thanks everyone for your advice; we'll try out the suggestions and let you know the results. Much appreciated! Now I get the 'Loads Of Trouble Usually Serious' thing!

[Keith Scarfe]

Another thing to check is that the tank has adequate breathing. I had a similar problem once on a race car (race cars suck out the fuel much quicker) when I changed the filling tube/cap and inadvertently did away with the breather. The car ran fine when cold or on idle but when driven, the fuel is sucked out and a vacuum builds up in the tank and it cant suck the fuel out anymore. Discovered when removing the filling cap and it was obviously under vacuum (straight away after stopping as the vacuum will slowly dissipate). Fitted separate breather (with one way valve) and it was ok after that. Check your breather is working well.

[worzel]

Hi

Now this is a long shot- but here goes.

Does the misfire occur after a certain period of time has elapsed- regardless of how far you've driven?

Do you have any sort of immobiliser fitted?

I ask because I used to get pretty much the same problem on the Excel I had. I'd start up and inevitably after around 20 minutes or so running the car would suddenly die (actually it developed an odd idle pattern) resulting in the engine slowing right down as the throttle was opened.

If I left the car for around a half hour it would restart and run fine until another 20 minutes or so had elapsed.

The cause- the immobiliser (a Cat 1) had been wired into the ignition circuit (by a qualified auto electrician incidentally). It turned out that the unit definitely wasn't designed to have current continuously passing thru it. A simple switch over to the starter circuit cured the problem.

Generally if s problem like the one you describe suddenly occurs then suspect the electrics. If it comes in gradually I'd look at the fuel system.

Regards

John

[Hapless Biker]

Cheers, no immobiliser on the car (sadly as that would have been an easy fix!) or inline filter. We?ll check out the breather issue noted by Keith and the techno, misfire sync that Rohan suggested; good places to start. Thanks all for your help, will keep you posted

[collins_dan]

The anti-theft switch is on the inside of the glovebox. You may have one and not know it. You could run a new line from the ignition switch to the coil, bypassing the tach and the anti-theft switch, allowing you to eliminate both from the equation. Dan

[Hapless Biker]

Cheers Dan, the anti theft switch has already been bypassed but running a new line bypassing the tacho might help, will give it a go. Cheers

[Hapless Biker]

BTW, this is the car causing all the trouble!