Engine misfires very intermittently

[Struggler]

Car will be fine for,
then all of a sudden it will feel very limp and then the back back to normal a second later.

If it happens in first or second gear, you go kangarooing (SP) off down the road,

basically it seems the engine is not firing on either one or two cylinders for a few seconds,
but then the problem disappears,

its happened while sitting in traffic (idle just drops and car shudders)
while pulling away, and while accelerating though 3rd gear (50-60)

any ideas what it could be??
ive changed the plugs for some nice bosh ones,
took the distributor off and cleaned it up,
cleaned up the HT leads,
then took her out for a rather hard drive,
initially the problem came back once or twice,
but then it seemed to go away for good......................for now,

but what else should i be looking at??
coil??

its an F7R engine btw

 

[Struggler]

please help!
i changed the spark plugs and went of a long drive,
(few short blasts of acceleration)

and the problem seemed to clear,

however i went over a pot hole last night and its started again!
seems to be mainly in the wet!

 

[S76Pilot]

TDC sensor or coil pack. Coil pack is £25 off ebay and 5 minutes to change and would rule that out. Known to cause problems. TDC sensor is fairly easily cleaned.

 

[Struggler]

its a valver mate, no coilpack on it,

ive cleaned all the distributer out though,

i was thinking TDC sensor last night, i may have a look today!
cheers

 

[Dave.]

have you made sure the spark plug wells are not getting water in to them? Can make it miss terribly!

 

[Struggler]

how would the wells get water in them??
where can i check??

 

[MicKPM]

Oh it happens more oftern than you know but in the case of the F7P/R-7XX engines its normally oil in them caused by leaking cam covers or cam carriers... new gasket and/or Loctite 518 is your friend

Mick

 

[Struggler]

Oh it happens more oftern than you know but in the case of the F7P/R-7XX engines its normally oil in them caused by leaking cam covers or cam carriers... new gasket and/or Loctite 518 is your friend

Mick

so youre saying that oil is getting out of the camcover (rocker cover??) and into the sparkplug wells, causing them to not fire properly yes?
and a new gaskets should solve this??
thanks for the advice btw.

ive just noticed, it only happens badly when going over bumps,
hit a pothole and it kangaroos for a few seconds.

 

[Dave.]

Could be a combination of the two, all I know is mine ran like a bag of nails due to me stupidly hosing down the engine bay!

Had to take leads out and soak up the water, then remove plugs and give them a clean and then dry out the leads. Put it back together and was 100% again

 

[Struggler]

ill give this a go,
but i cant see how the plugs would have got any water on them,
theyve only been in there a week.

also going to clean up the TSP, TDC and coilpack.
then move on to replacing them all along eith the HT's

got to love mk1 ownership!

 

[Struggler]

Anyone got any more advice on this??

so far ive replaced the spark plugs,
cleaned the distributor cap,
cleaned the coil pack
clean the TPS
checked the earth straps

and the problem persists,
im going to have it plugged into a fault reader this weekend to see if it comes up with any results.
then im going to proceed to replace the following.

TPS sensor
TDC sensor
coilpack
HT leads
distributor cap
gearbox earth straps,

and if all this fail,
im just going to curl up in a ball and cry!

 

[Silent_Scone]

if it's reallllly intermittent I'd put money on oil in plug wells, I had a leaky VVT Solenoid which was causing a slight misfire every so often. So cam cover reseal would be my guess

 

[Struggler]

that would may a bit of sense, as i cleaned the plugs up among other things, and the problem went away for a week.
however when i pulled out a HT lead, the wells looked pretty empty of oil (though i only did this with one)

its very intermittent, going along a smooth bit of road and it wont happen at all,
go down a bumpy road and it will happen lots.

ill take the plugs out tomorrow and clean them up if need be, but how do i clear the wells of oil??

 

[MicKPM]

Right, there's several ways to "clear" the plug wells of oil... some approved, others more "pikey". Firstly pull each lead out as it's not hard to visually inspect these engines for leaks. If there's any sign of a lead end looking like a dipstick you'll know there's a leak. Have a good look down the business end of the barrel - See anything other than shiny Aluminium? If there is any sign of fluids you can either mop it up with paper towels which does take a while but persevere with it or you could be a pikey and let it drain to the bore (Be carefull if you do this as, depending on volume found, you can hydro-lock the lump and cause all manor of destruction)

If its not being caused by oil in the plug wells (or water for that matter) then the problem is most likely down to something daft like a breaking down lead (Possibly the king lead if it only happens when you go over bumps as it's the only one that can really flex seeing as its 'free'), Old/Worn igntion component such as distributor cap and rotor arm. Possibly, another issue could be with the coolant temp sensor for the ECM giving a false signal but if that were the case the missfire would most likely be a bit more predictable though I doubt diagnostics will reveal much on an old 12 pin setup.

You mention above that you've "cleaned" the TPS... as in the Throttle Position Sensor? If so, did you unscrew it from the throttle body to do this?

Mick

 

[Struggler]

when looking into the spark plug wells,
i cant see any oil at all, ill have a more thorough look when i get the chance

ill take the plugs out and let anything that is there drain into the engine, and see if that changes anything

and yes, i took it out, checked it still moved, cleaned up the connection and put it back in
one wire going to it was very frayed, no still in tact.

where a outs is this "king Lead"?
i fear if its a breaking down lead somewhere in the engine, i may never find it at all!!

 

[MicKPM]

Well that's good then if you can't see any fluids in the plugwells. The King Lead is the main lead from the Ignition module to the centre of the distributor cap. Have you checked the condition of the distributor car and rotor arm yet?

Uh-oh.... The TPS position is factory set as there's a fair bit of adjustment in them on the Mk1. Its not the cause of your problems "as is" but it proberbly won't help that you've moved it as they can give problems.

 

[Struggler]

yes, ive checked the condition of the distributor cap and arm,
no obvious sign of any problems there, i cleaned up the connections anyway,

i plan on replacing the cap and all the HT lead for uprated ones.

why is the TPS adjustable?? i did mark it and make sure it went back in the same place either way.
will getting this very wrong cause more problems?? im not really too sure what the TPS sensor is used for!

 

[MicKPM]

The "TPS" or "Throttle Position Sensor" transforms the rotary movement of a throttle plate to a resistance value which the ECM then translates to an electroninc throttle position as a % from 0% (Being shut) to 100% (Being fully open). From this reading it knows how to work out the fuelling requirements and advance/r****d the igntion accordingly for the engine to run smoothly. There is no real "Adjustment" in the sensor position. They're fitted at factory and allow for a certain movement to ensure they're set/sat perfectly central on the spindle at the time of production - afterwards, the min and max ranges are set electronically and programmed to the ECM. After this the sensors location itself is then marked with paint on the throttle body before leaving the factory for storage or dealership PDI (The paint acts as an anti-tamper marking)

If you've moved it and you replace it slightly out of alignment or off the original position the computer doens't know it's been moved and so the throttle plate reading will be wrong...

example: actual throttle plate position 38% Vs measured throttle position 24%

... This kind of erroneous reading can cause a misfiring and poor running condition as the ECM assumes the TPS is correct and so runs the engine accordingly. Still, I don't think this is your problem but you asked what a TPS is so

Mick

 

[Struggler]

well i took it off,
but it had a very distinctive mark from where the screw had sat against it for all those years,
so i put it back exactly on that. and it runs perfectly!

thanks for the explaination though mick