Misfire - bet you've no idea...
26-04-2010, 05:37 PM
#1
Misfire - bet you've no idea...
Acquired an absolutely stunning XR3i for my 21st recently. It's 89 on EFI. Covered 39k miles from new. Completely original.
It has a niggly misfire on idle and light throttle, up until about 2500rpm, on half throttle or more it's ok wherever the rev's are. It's perfect on first cold start then starts to miss after a few minutes of warming up.
My dad thought maybe a dodgy/sticky valve at first. It's had new leads in past week or so just before i got it.
We took it to ProTec motorsport in preston after our workshops couldnt work out what was up. They reckoned it was a burnt valve on cylinder 1, which was what my dad assumed.
Had all 4 exhaust valves replace for new, along with a full set of hydro lifters, gaskets etc... This was only 4 days after having full cambelt and filters service.
Exactly the same after being built back up! Still on cylinder 1 too. So i swapped the injectors on 1 & 4 round and the problem moved. Its less on cylinder 1 now, and not definate on cylinder 4. I'm pulling my hair out!
One suggestion was the mixture potentiometer. Apparently the hydrocarbons were way off, which is why the guys at ProTec thought a burnt valve not seating correctly.
Help?
It has a niggly misfire on idle and light throttle, up until about 2500rpm, on half throttle or more it's ok wherever the rev's are. It's perfect on first cold start then starts to miss after a few minutes of warming up.
My dad thought maybe a dodgy/sticky valve at first. It's had new leads in past week or so just before i got it.
We took it to ProTec motorsport in preston after our workshops couldnt work out what was up. They reckoned it was a burnt valve on cylinder 1, which was what my dad assumed.
Had all 4 exhaust valves replace for new, along with a full set of hydro lifters, gaskets etc... This was only 4 days after having full cambelt and filters service.
Exactly the same after being built back up! Still on cylinder 1 too. So i swapped the injectors on 1 & 4 round and the problem moved. Its less on cylinder 1 now, and not definate on cylinder 4. I'm pulling my hair out!
One suggestion was the mixture potentiometer. Apparently the hydrocarbons were way off, which is why the guys at ProTec thought a burnt valve not seating correctly.
Help?
26-04-2010, 07:12 PM
#2
Second thoughts, think i've narrowed it to a bloody MAP sensor!!
When idling and up to temperature, very rough, if i unplug map sensor it evens out much better and is almost fine. With sensor unplugged each cylinder goes down equal to the rest when lead is pulled. Which suggests to me the injectors are all firing equal without map sensor, instead of some firing more than others. With sensor plugged in i can hear a slightly louder ticking noise which i assume to be an injector working harder to even out.
I'll get a sensor in morning and try it, if it runs better its having fine tune/set up and should be sorted...
When idling and up to temperature, very rough, if i unplug map sensor it evens out much better and is almost fine. With sensor unplugged each cylinder goes down equal to the rest when lead is pulled. Which suggests to me the injectors are all firing equal without map sensor, instead of some firing more than others. With sensor plugged in i can hear a slightly louder ticking noise which i assume to be an injector working harder to even out.
I'll get a sensor in morning and try it, if it runs better its having fine tune/set up and should be sorted...
26-04-2010, 08:40 PM
#3
Too many posts.. I need a life!!
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Hi Mate,
I would definitely have done a compression test before dismantling the engine to change valves etc as it is unlikely to be a mechanical problem. I would have expected a good garage to do this straight off the cuff. In fact I suggest you compression test it before doing anything else.
However, I think this is quite likely to be an electrical issue. I doubt its the MAP sensor as this would not affect one injector. By running it with the MAP sensor unplugged you'd be putting the ECU into limp home mode which could change everything. It might be richening up the mixture and hiding the problem caused by something else.
Have you tried unplugging the ECU to reset the KAM memory? Also - it might be worth getting one of those Ł25 Gunson fault code readers from Halfords as this might help shed some light on it. I would also look at testing both the air and water temperature sensors using a voltmeter.
The potentiometer is really used for adjusting idle mixture and is unlikely to cause a misfire whilst driving.
When you say you need a fine tune/set up, what are you talking about doing? Being EFi very little apart from the idle mixture is adjustable - everything else is fixed by the ECU. I'm sure someone would take you money for a "setup", but in reality they wouldn't be doing much at all.
I would definitely have done a compression test before dismantling the engine to change valves etc as it is unlikely to be a mechanical problem. I would have expected a good garage to do this straight off the cuff. In fact I suggest you compression test it before doing anything else.
However, I think this is quite likely to be an electrical issue. I doubt its the MAP sensor as this would not affect one injector. By running it with the MAP sensor unplugged you'd be putting the ECU into limp home mode which could change everything. It might be richening up the mixture and hiding the problem caused by something else.
Have you tried unplugging the ECU to reset the KAM memory? Also - it might be worth getting one of those Ł25 Gunson fault code readers from Halfords as this might help shed some light on it. I would also look at testing both the air and water temperature sensors using a voltmeter.
The potentiometer is really used for adjusting idle mixture and is unlikely to cause a misfire whilst driving.
When you say you need a fine tune/set up, what are you talking about doing? Being EFi very little apart from the idle mixture is adjustable - everything else is fixed by the ECU. I'm sure someone would take you money for a "setup", but in reality they wouldn't be doing much at all.
28-04-2010, 09:39 AM
#4
Thanks for the response dude.
We did a compression test before having valves done and it seemed ok if a little low on number 2. But some were burnt a bit after removing so wasn't helping. And couple lifters were noisy.
I figured if the map sensor thought it was seeing more air than it was then it'd inject more fuel to compensate. Giving the slightly rich running it seems to be doin. Protec said the hydrocarbons were all over the place. Compression is spot on both hot and cold with no leaks. Spark appears good too.
My next thought was to reset kam values as haynes said it could cause similar symptoms but didn't say how and read a couple ways on Internet but didn't want to just go ahead and fry ecu if they're wrong. Do ya literally just unplug it?
What values should I expect from air and water temp sensors when standing over night and when fan comes on?
Protec have the old sun diagnostics still which is why it's going back to them. They only had half hour last time before going rallying but got much longer today. Plus they owe us the time since it was 'wrongly diagnosed' and cost us a few quid at our workshops. (which to be fair is money well spent in the long run).
We did a compression test before having valves done and it seemed ok if a little low on number 2. But some were burnt a bit after removing so wasn't helping. And couple lifters were noisy.
I figured if the map sensor thought it was seeing more air than it was then it'd inject more fuel to compensate. Giving the slightly rich running it seems to be doin. Protec said the hydrocarbons were all over the place. Compression is spot on both hot and cold with no leaks. Spark appears good too.
My next thought was to reset kam values as haynes said it could cause similar symptoms but didn't say how and read a couple ways on Internet but didn't want to just go ahead and fry ecu if they're wrong. Do ya literally just unplug it?
What values should I expect from air and water temp sensors when standing over night and when fan comes on?
Protec have the old sun diagnostics still which is why it's going back to them. They only had half hour last time before going rallying but got much longer today. Plus they owe us the time since it was 'wrongly diagnosed' and cost us a few quid at our workshops. (which to be fair is money well spent in the long run).
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