Strange Missfire

[fliprio 9]

I have a bit of a weird misfire, generally only when hot, under load anywhere between 4 and 7k, sometimes it’s like a small hesitation, other times its more like a backfire and the car hesitates.

 

It’s on Carbs and I’m fairly certain the fuelling is out, it was tuned on the rolling road a few months back, but has since started to use lots of fuel and showed 8% co on the MOT, idle has dropped as well. Can the misfire be down to overfueling? I didn’t want to take it all the way down to the Rolling road again for them to tell its misfiring and can’t do a proper session on it.

 

Since it only seemed to do it when hot, I tried a different ignition module, which didn’t help at all

 

Any ideas?

[harryskid 193]

You have not said what engine you have!

[fliprio 9]

You have not said what engine you have!

 

woops, its a GTi-6 running on the 1.9 distributor and twin 45 carbs (no ECU or anything like that)

[fliprio 9]

I have a bit of a weird misfire, generally only when hot, under load anywhere between 4 and 7k, sometimes it’s like a small hesitation, other times its more like a backfire and the car hesitates.

 

It’s on Carbs and I’m fairly certain the fuelling is out, it was tuned on the rolling road a few months back, but has since started to use lots of fuel and showed 8% co on the MOT, idle has dropped as well. Can the misfire be down to overfueling? I didn’t want to take it all the way down to the Rolling road again for them to tell its misfiring and can’t do a proper session on it.

 

Since it only seemed to do it when hot, I tried a different ignition module, which didn’t help at all

 

Any ideas?

 

I have now tried a different coil and ignition pack and its made no difference, seems to get progressily worse, about 6 miles into a trip it will make small hesitations, and after about 8 miles it will start to missfire, only ever does it like this, at high revs, driving around normaly is fine.

 

anyone experienced anything like this?

[fliprio 9]

I have now tried a different coil and ignition pack and its made no difference, seems to get progressily worse, about 6 miles into a trip it will make small hesitations, and after about 8 miles it will start to missfire, only ever does it like this, at high revs, driving around normaly is fine.

 

anyone experienced anything like this?

 

someone must have an idea of other things I could try?

 

im baffled as to why it only does it when hot

[wracing 19]

Sounds heat related, getting progressively worse as you drive. so could be a manor of things. incorrect fuelling can cause the symptoms as can faulty ignition components. Try replacing leads dizzy cap plugs rotor arm, and see if you have any improvement.

 

James

[fliprio 9]

Sounds heat related, getting progressively worse as you drive. so could be a manor of things. incorrect fuelling can cause the symptoms as can faulty ignition components. Try replacing leads dizzy cap plugs rotor arm, and see if you have any improvement.

 

James

 

cheers for the reply James

 

would the leads or cap cause the issue only when hot? I thought that this was sometimes more of a problem when the car was cold because of moisture in old HT leads.

 

My other problem is which leads to get, its on a GTI-6 conversion, so doesnt have the normal 205 leads. Anyone got any ideas on what leads these need as I imagine the orginal injection set-up had individual coil packs??

[mickie 3]

mine does this above 5k. im putting it down to carbs too running lean

[fliprio 9]

mine does this above 5k. im putting it down to carbs too running lean

 

I had mine set-up on a rolling road and all was fine for a few months, something seems to have happened to them and its not running right, I had thought the mixture above 3k on the webber type carbs was down to the jet size (which hasnt changed on mine) and below 3k you use the adjuster screws for the primary jets. which means the only thing that could cause it to run lean is lack of fuel supply at high RPM (i could be completely wrong though ). Hopefully taking it to another RR soon to try and diagnose the problem.

 

has anyone done the GTI-6 conversion on a dizzy, any ideas on what leads you used?

[Cameron 16]

Have you had the carbs set up, or did you just fit them and hope for the best?

 

What ignition advance are you running?

 

It sounds like pre-ignition brought on by either lean mixture or incorrect ignition timing (or both).

[brumster 135 1 Cars]

Does looking at the plugs give you any hints? Might suggest which carb/lead/plug to be looking at, if nothing else... could be a breaking-down plug or lead. Must admit, temperature-related makes me wonder how the carb could be contributing, unless it's fuel starvation or 'boiling' in the float bowl, but that sounds a little extreme...

[jackherer 543]

Was the replacement ignition amp you tried definitely a known good one? Was it on a heatsink plate with thermal transfer paste applied?

[Cameron 16]

The risk of pre-ignition increases as engine temp rises, so it could be that it's ok when cold and warming up but when it gets up to temp it's just that little bit too lean / too much advance.

 

He isn't using the ECU so it can't be anything to do with coolant temp sensors. It runs fine when it's cold which makes me think it can't be a fault with any of the ignition components, it has to be a set-up issue. If the coil or module was knackered then it would be misfiring all the time, surely.

[brumster 135 1 Cars]

I had an amp-related problem that would only occur once some heat was in it, so it's sensible to suspect (although the OP has replaced it once already, of course - assuming he replaced it with a good one!). But I agree, could be a multitude of things really I would check the entire ignition system first though, simple because it's easier to diagnose problems in these bits yourself - euurgh, I hate carbs

[Cameron 16]

Yep! Would have been better off sticking with the standard management.

 

I'm interested to see replies to my first questions though.. if the carbs have just been bolted on and not set up then it's not very surprising that he's having running problems.

[fliprio 9]

Yep! Would have been better off sticking with the standard management.

 

I'm interested to see replies to my first questions though.. if the carbs have just been bolted on and not set up then it's not very surprising that he's having running problems.

 

 

carbs were set-up on the rolling road about 1000 miles ago, all running fine and gave 170 bhp, which is almost standard power for the engine. The dizzy advance was backed off a bit, still pinks between 2 to 3k if using a high gear under load, but it was the best compromise the RR guy could get..

 

it recently went in for its MOT and was running 8%co and has started using a lot of fuel, so i suspect that the carbs have gone out of adjustment some how.

 

The ignition amp was a known working amp, but handnt been used in a while and was from my local friendly peugeot specialist on loan, as with the coil. He said to use copper grease on the back of it rather than heatsink paste, although there was nothing on the orginal one at all!

 

just replaced the HT leads and dizzy cap as the history of them isnt known as I have only had the car for 6 months.

[fliprio 9]

I had an amp-related problem that would only occur once some heat was in it, so it's sensible to suspect (although the OP has replaced it once already, of course - assuming he replaced it with a good one!). But I agree, could be a multitude of things really I would check the entire ignition system first though, simple because it's easier to diagnose problems in these bits yourself - euurgh, I hate carbs

 

the second amp did it at exactly the same time (distance from driving) than the orginal, this was my first thought after reading a lot of hot missfire related posts

[Cameron 16]

So has this all started happening around the same time it started using a lot of fuel?

[fliprio 9]

So has this all started happening around the same time it started using a lot of fuel?

 

yes, although i thought rich mixture wouldnt cause it to missfire/hesitate like it is

 

to add into the mix, it kept concking out on me and you could hear the fuel pump kick in and out, turned out the fuel pump relay was a bit intermittent, so have replaced that and that side of things seems to be fine now, with the fuel pump being heard constantly.

[fliprio 9]

yes, although i thought rich mixture wouldnt cause it to missfire/hesitate like it is

 

to add into the mix, it kept concking out on me and you could hear the fuel pump kick in and out, turned out the fuel pump relay was a bit intermittent, so have replaced that and that side of things seems to be fine now, with the fuel pump being heard constantly.

 

 

the fueling was found to be spot on when I got it on the rolling road yesterday, a little lean at very light throttle, which causes a bit of jurkyness on light throttle, but wouldnt cause it to missfire. i replaced the dizzy cap, rotor and leads and it hasnt missfired since, so hopefully this was the cause and the problem isnt hiding for a few days.

 

i had also been running it on 95 octain fuel rather than the 97 that it was tuned un on the rolling road with, weather that makes a difference i dont know.

[Cameron 16]

Yeah that will definitely make a difference! If the engine was tuned on 97 RON then it could have been mapped with more ignition advance to make the most of the fuel. This advance could be too much for the lower octane fuel causing it to knock, especially at higher temperatures.

 

Make sure you always run the fuel your engine was mapped on! If you have to use lower octane fuel, make sure you only do it as an emergency get you home fix, and fill it up with proper fuel as soon as you can.

Edited October 9, 2010 by Cameron