Discussion
Hi All, need some help/advice
I have a misfire that has developed quite suddenly over the past week. Only significant thing was a short period of high temp running and idling.
The car starts and idles well at all temps.
From cold there is a subtle flat spot at part throttle...nothing audible you just kind of know from your right foot that there should be more.
Once warmed up, there seems to be a trigger point (temp? could be, could also be faster running, not sure) where things get ugly.
If you accelerate really gently then not that noticeable. If you try and accelerate normally you hit a wall, nasty loud rumbling shaky misfire. If you floor it you can sort of drive through pretty well...almost like a magic point where throttle opening and fuel delivery is enough to just swamp the problem and off it goes. On the Mway once cruising in 5th, it pull fine but you can still hear a misfire rumble. I have checked by going through gears etc that it has no obvious link to TPS position or engine revs as-such...it can be horrible at 1500rpm in 4th, 3500rpm in 4th and 3500rpm in 2nd etc etc.
Here's what I know/have done:
1. Changed HT leads...nope
2. Changed coil pack...nope
3. Checked plugs...all look rather lean...not severe, just not much light brown to see. All 6 very similar even after idle
4. Compression test shows 220ish psi on all cylinders
5. If I use a hand over exhaust tip, the left one (rear bank) has a definite random strong puff-pop i.e. misfire. Right hand has nothing and is definitely quieter than the left side overall
6. Smelling the fumes, left is almost nothing, right has that very mild but slightly sweet burnt petrol smell that I'd expect
7. If I pop off each injector plug one at a time, each one has an effect on the running, so all are doing something
8. If I pop off any from 4,5,6 the left pipe makes a strong regular puff-pop misfire in time with engine cycle and interestingly then doesn't seem to have a random one superimposed on top...may just be masking it I suppose, but odd
9. I have checked the injector wiring and voltages with DMM, all look fine (thinking of the corroded wiring loom found on this forum some years back)
10. Diagnostics seem ok-ish but adaptives do look a bit low, heading to -10% ish and did notice rear bank heading down to -20% at one point. Seems at odds with plug colours...if it was correcting a rich mixture then surely it would tend to look rich on the plugs...dunno, perhaps not
11. Diagnostics show both lambdas working and indeed, if I unplug one injector briefly on a bank, that bank's adaptive's change
12. Idle is "fine" all through this (obviously there is a misfire as described) and there is no popping in the throttle bodies etc, so dont think there's an air leak
So now I'm wondering if I could have an injector problem, or even overall fuel delivery problem ? For sure it feels like fueling to me. The fact that I can press the gas peddle hard down and blast through the problem might be significant.
That's war-and-peace complete. Any ideas folks? I'm into unknown territory.
I have a misfire that has developed quite suddenly over the past week. Only significant thing was a short period of high temp running and idling.
The car starts and idles well at all temps.
From cold there is a subtle flat spot at part throttle...nothing audible you just kind of know from your right foot that there should be more.
Once warmed up, there seems to be a trigger point (temp? could be, could also be faster running, not sure) where things get ugly.
If you accelerate really gently then not that noticeable. If you try and accelerate normally you hit a wall, nasty loud rumbling shaky misfire. If you floor it you can sort of drive through pretty well...almost like a magic point where throttle opening and fuel delivery is enough to just swamp the problem and off it goes. On the Mway once cruising in 5th, it pull fine but you can still hear a misfire rumble. I have checked by going through gears etc that it has no obvious link to TPS position or engine revs as-such...it can be horrible at 1500rpm in 4th, 3500rpm in 4th and 3500rpm in 2nd etc etc.
Here's what I know/have done:
1. Changed HT leads...nope
2. Changed coil pack...nope
3. Checked plugs...all look rather lean...not severe, just not much light brown to see. All 6 very similar even after idle
4. Compression test shows 220ish psi on all cylinders
5. If I use a hand over exhaust tip, the left one (rear bank) has a definite random strong puff-pop i.e. misfire. Right hand has nothing and is definitely quieter than the left side overall
6. Smelling the fumes, left is almost nothing, right has that very mild but slightly sweet burnt petrol smell that I'd expect
7. If I pop off each injector plug one at a time, each one has an effect on the running, so all are doing something
8. If I pop off any from 4,5,6 the left pipe makes a strong regular puff-pop misfire in time with engine cycle and interestingly then doesn't seem to have a random one superimposed on top...may just be masking it I suppose, but odd
9. I have checked the injector wiring and voltages with DMM, all look fine (thinking of the corroded wiring loom found on this forum some years back)
10. Diagnostics seem ok-ish but adaptives do look a bit low, heading to -10% ish and did notice rear bank heading down to -20% at one point. Seems at odds with plug colours...if it was correcting a rich mixture then surely it would tend to look rich on the plugs...dunno, perhaps not
11. Diagnostics show both lambdas working and indeed, if I unplug one injector briefly on a bank, that bank's adaptive's change
12. Idle is "fine" all through this (obviously there is a misfire as described) and there is no popping in the throttle bodies etc, so dont think there's an air leak
So now I'm wondering if I could have an injector problem, or even overall fuel delivery problem ? For sure it feels like fueling to me. The fact that I can press the gas peddle hard down and blast through the problem might be significant.
That's war-and-peace complete. Any ideas folks? I'm into unknown territory.
so think we can rule that out.OK, that's what I thought you meant. I did the compression test at quite hot engine temp. I think that would have shown something. But for sure valve clearance test is looking closer and closer.
Just been playing and using old diags to watch the lambdas...both seem normal but there's deffo a kind of cyclic effect going on when it riches up, both lambdas go high, adaptives go more -ve, then both lambdas ping pong for a few secs then they both settle low for a few secs, then they ping pong again for a while, it riches up again etc etc. Engine rpm follows the same cyclic effect changing by perhaps 50rpm (audiable)...which could be the cause or the effect of course.
Also now got new plugs. Same.
Do injectors ever fail ??? Really ? I run on premium unleaded btw.
Just been playing and using old diags to watch the lambdas...both seem normal but there's deffo a kind of cyclic effect going on when it riches up, both lambdas go high, adaptives go more -ve, then both lambdas ping pong for a few secs then they both settle low for a few secs, then they ping pong again for a while, it riches up again etc etc. Engine rpm follows the same cyclic effect changing by perhaps 50rpm (audiable)...which could be the cause or the effect of course.
Also now got new plugs. Same.
Do injectors ever fail ??? Really ? I run on premium unleaded btw.
My misfire is pretty nasty, would not want to drive on it too much. Its like a small earthquake under your right foot (the whole car vibrates) and the amount of "go" is very poor...until you press your foot down further and then off it goes with gusto. Not saying the misfire has gone when you press on, just that you suddenly get most of the acceleration back (quite specifically when you have opened the throttles pretty wide, there it is!).
Mine fault started after some hours of tinkering when the engine got really hot. Soon after that it started. Up til then it had been behaving really well!
You've got to luv 'em!
I'm pretty convinced its fueling related. The fact the rear bank adaptives are v low and kind of cycling is suspicious.
Mine fault started after some hours of tinkering when the engine got really hot. Soon after that it started. Up til then it had been behaving really well!
You've got to luv 'em!
I'm pretty convinced its fueling related. The fact the rear bank adaptives are v low and kind of cycling is suspicious.
Cats look pristine...no sign of meltage or damage. Also changed the crank sensor just in case...no change.
Tried running with lambdas disconnected ....still misfires. Only slight clue after that test is that plug 6 and 5 look a tiny bit rich and the other 4 look like they just came out of the packet.
Injectors arrive end of next week.
Running out of tests now.
Tried running with lambdas disconnected ....still misfires. Only slight clue after that test is that plug 6 and 5 look a tiny bit rich and the other 4 look like they just came out of the packet.
Injectors arrive end of next week.
Running out of tests now.
Hi John
Really appreciate your thoughts on this...always good to get your input.
The non-volatile RAM chip is new as of around 2 years.
The fuel regulator...I dont think it does much so doubtful on that. Seems hard to fathom that it would block or restrict flow.
The throttle body seal had occurred to me too but they've been in place for about 2 years now with rtv...which has always worked for me. I dont bother with gaskets. There is plenty of vacuum on all cylinders when putting hand over each body mouth. Also in my past experience you always get pops through the bodies when there's a leak...and there is nothing. But it is "on the list"
The only reason I think its fuel is a feeling...you know when you get the throttles out of balance and at first-press of throttle you can get that kind of roughness sensation...it feels a bit like that. Its hard to know if its just one cylinder. Its almost like there is too much fuel and when you open the throttles enough you get back to approximately correct mixture and off it goes.
One other random thought was whether one of the cams has moved relative to its sprocket...i.e. cam timing all to cock...*very* unlikely but also needs to be checked.
The fact it still misfires without lambdas connected seems to rule out the lambdas...obviously
Well, I suppose its obvious, may surprise me yet!
Gotta luv 'em
Really appreciate your thoughts on this...always good to get your input.
The non-volatile RAM chip is new as of around 2 years.
The fuel regulator...I dont think it does much so doubtful on that. Seems hard to fathom that it would block or restrict flow.
The throttle body seal had occurred to me too but they've been in place for about 2 years now with rtv...which has always worked for me. I dont bother with gaskets. There is plenty of vacuum on all cylinders when putting hand over each body mouth. Also in my past experience you always get pops through the bodies when there's a leak...and there is nothing. But it is "on the list"
The only reason I think its fuel is a feeling...you know when you get the throttles out of balance and at first-press of throttle you can get that kind of roughness sensation...it feels a bit like that. Its hard to know if its just one cylinder. Its almost like there is too much fuel and when you open the throttles enough you get back to approximately correct mixture and off it goes.
One other random thought was whether one of the cams has moved relative to its sprocket...i.e. cam timing all to cock...*very* unlikely but also needs to be checked.
The fact it still misfires without lambdas connected seems to rule out the lambdas...obviously
Well, I suppose its obvious, may surprise me yet!Gotta luv 'em
Injectors made no difference. I fitted Lucas D1720AB types which race Tech list for the 4.0. The originals are D1000AB which they only list for the 3.6 very odd. Anyway it runs albeit adaptives are trying lean it out on both banks so I suspect these are the higher flow injectors. But either which way...it still misfires exactly the same. Also does the same with lambdas unplugged. Next check is cam timing...could it have slipped. It's the only mech thing left to check that I can think of. BTW, stethoscope on each cylinder they all sound the same. None have any obvious stumbles etc.
The one at the point of problem starting was brand new as of perhaps 2 years ago when I was trying to improve shunting. It made no difference at all. The one I tried this time round was the original, hence pretty confident its ok. My buddy is convinced its electrical having experienced it. Running on 5 cyls...how rough is it ? I would guess not *that* bad, just a bit rough but still quite a bit of go ? When I'm in the "zone", modest extra throttle makes almost no extra accel, it just gets noisier, shakes more, until you plant you foot and off it goes (couldn't guarantee there isn't still a misfire in the background, but it certainly gets most of its power back). His theory goes that anything mechanical will get worse at load and speed, and I think I take his point.
It's all back together with the original injectors now ready to start some buzzing out.
Interesting that after a short drive with the new injectors...all plugs look pristine but 6 is pretty sooty. Guess the ecu can't lean it out enough with those injectors. But strange that 4 and 5 look perfect! How could 6 be different unless it's missing some spark.
Interesting that after a short drive with the new injectors...all plugs look pristine but 6 is pretty sooty. Guess the ecu can't lean it out enough with those injectors. But strange that 4 and 5 look perfect! How could 6 be different unless it's missing some spark.
Gassing Station | Speed Six Engine | Top of Page | What's New | My Stuff