Misfire under acceleration - Only when warm!

[72elan1304]

As the title says, My S1 135 has started to misfire.

It only does it once warm, after about half an hour of running, and only when accelerating. Once up to speed, at steady it throttle is runs fine.

I've checked the leads and the distributor and all look fine.

Any ideas?

[dirkpitt]

umm no expert but check the condition of your spark plug?.....robin+co will be along soon to put you right!

[72elan1304]

Plugs were changed less than 1000 miles ago.

[robin]

Did it misfire before plugs were changed?

Does the idle speed behave normally - i.e. fast idle when cold, slower idle once warm?

Does the engine fan cut in if you run it up to temp then hold 2,500 RPM until either fan cuts in or you hit 102C indicated and chicken out?

Assuming the above is OK, my money would be on dodgy TPS or the new favourite, rotor arm

When cold the fuelling is rich due to cold running. When warm the engine would run lean on throttle transients were it not to see the throttle opening; it uses the throttle opening action to trigger some extra fuel for a few cycles, then reverts to closed loop (when appropriate) fuelling.

Can you accelerate to the "national speed limit" in 3rd gear when safe to do so ? Or does it start to hesitate/jump about above 4,000 RPM?

Cheers,
Robin

[72elan1304]

No misfire before the plugs were fitted, or for about 500 miles / 6 months after. It idles fine when either cold or warm. The misfire first happened after being parked up outside for a few weeks. In this time it snowed a few inches and melted. I also had to start the car using a secondary battery with jump leads.

There is a fault with the temperature read out, it under reads. Ed pointed this out when I bought it from him. The cooling fan still kicks in, but just at a lower indicated temp.

Rotor arm I know, but TPS?

Once warm, it will hesitate and mis under anything more than very gentle acceleration at any rev's.

[Stewart]

Rotor arm I know, but TPS?

Throttle Position Sensor. Sorry that is the extent of my knowledge

[steviej]

You could try checking and possibly closing the plug gaps, they often come 'pre gapped' at 40 thou + nowadays to suit modern ignition systems and the older systems cant cope with the bigger gaps so well, causing misfires.

[robin]

You can ignore the indicated temp as this is only for your benefit, the ECU has a second temp sensor that it uses for controlling fuelling and cooling and that sounds to me like it's working.

You can coax it to accelerate by opening the throttle a bit, waiting for the engine to settle into steady running, open throttle a bit further, etc. What I want to know is whether you hit a wall at 4K where you can no longer achieve this, i.e. the engine never settles into steady running once you've left the throttle where it should be.

If it's bad enough to misfire in neutral (i.e. no load) then you can test this in neutral - I don't need the car to be moving - sometimes you find that the misfire is only really noticeable when the engine is under load which is why I suggested 3rd gear.

My money would be on broken TPS wire or actual TPS fault. If you have a multi-meter you can try unplugging the TPS connector and testing the TPS. It has three pins. You should see a steady ~5K ohms between the outer two pins, no matter what the throttle angle. You should see between 0-whatever you measured before between the middle pin and either of the other two pins as you sweep the throttle angle from closed to wide open. The increase/decrease in resistance should be smooth and proportional to the angle (i..e half throttle should read about half the max resistance).

Then inspect the connector for corrosion in both the TPS itself and the loom plug.

Then tug each of the loom plug wires to make sure they're not fractured/coming away from the plug or crimp terminal.

Finally with the ignition on you should see +5v between the outside pins on the loom plug (or -5V if you happen to measure it back to front - doesn't matter). This should remain there even when you tug/wiggle the wires.


Cheers,
Robin

[robin]

You could try checking and possibly closing the plug gaps, they often come 'pre gapped' at 40 thou + nowadays to suit modern ignition systems and the older systems cant cope with the bigger gaps so well, causing misfires.

Though it did run fine on the new plugs for 500 miles, so not sure how likely this is.

The S1 K is very prone to getting water into the spark plug bores too which causes all sorts of misfire woe, though if I remember correctly they were not limited to acceleration.

[72elan1304]

Will have a poke about at the weekend at get back to you.

Thanks for the pointers

[dirkpitt]

umm thinking from an other angle ..if it isnt electrical ...then it would be fuel related?....maybe there is a blockage, from some dirt getting into the fuel system...eg dirty filter sock in tank, this could cause a bit off fuel starvation under sudden acceleration and hence a misfire effect ...or maybe air getting into the fuel line some where.... worth having a look....try some injector cleaner in the tank to see if that may help clean through ......

Also, check the induction filter and MAF to make sure they are nice and clean and not restricting...again sudden lack of air flow can cause the engine to starve and you get a misfire like sensation, or a jolting, but just a thought…....M2P, for what its worth

[robin]

There is no MAF on this car - it uses a MAP and that is integral to the ECU (fed via a black/white rubber tube from the intake manifold).

Fuel starvation would only make sense if it the car couldn't maintain high revs; the extra fuelling for acceleration is relatively small compared to the load-based fuel.

Air intake filter blocked would also cause hesitation on acceleration but usually results in a rich mixture which is less likely to cause misfire; most of the load calculation in the ECU is done on manifold pressure; a dirty filter looks no different to a less-open throttle from this point of view; throttle transients cause extra fuel to be added open-loop, but the extra fuel that is added is done so as a fraction of current load-based fuel, so you can only get so much extra fuel in.

It's cheap to check the air filter, though, so no harm in doing it.

Injector flush costs money so I wouldn't just bung it in unless you thought there was an injector issue (and then I might be more inclined to remove injectors and send away for cleaning).

Cheers,
Robin

[72elan1304]

Took the car out today, drove for a half an hour or so with no problems. Had to stop and let the car idle for a good while to push the temp up before it started missing. Wasn't as bad as before though. Cleared itself within a minutes driving.

Checked the TPS connections, photo's are below. Look fine to me, no corrosion. The wires are soundly connected to the back of the plug. I forgot to borrow the meter from work to check the readings.

Would it be worth changing the TPS as a precaution? Or is it as likely to be a faulty plug, HT lead, distributor etc.


Untitled by sandyurq2, on Flickr


Untitled by sandyurq2, on Flickr

[robin]

It won't be a faulty distributor though it could be a faulty rotor arm.

Assuming your car has the plastic manifold I think it's unlikely that the TPS has failed in this particular way (too intermittent).

Are you losing coolant?

Cheers,
Robin

[r055]

I sound like a broken record, but I had same problem with mine after the plugs an leads were changed over. Turns out that it was the routing of the very slightly longer leads were just off and interfering with a sensor.

Makes sure they route perfectly in a straight line and if there are any slack keep it up at the plug end rather than mid route. Very simple fix you should rule out first before being too technical!