Misfire at idle and when holding around 2000rpm.

Slunch

New Member
As above.

I pulled of the downpipe for repairs and fitted a new rad. When warming up the car to check for coolant leaks there was a really noticable misfire at idle, almost like a stumble from race cams. I think it's been there for a while, I just haven't noticed. This persisted when revs were held at 2000 rpm, didn't sound like race cams anymore, but appeared to clear when the revs were above this. At idle AVCR is telling me that the revs are oscillating between 900 and 950 in line with the misfire.

So far I have checked plugs, nice mid brown electrode, fair bit of soot on the case. Gapped at .75mm

Old fuel dropped and replaced with fresh 99.

Leads seem ok, no water in the wells etc.

Injectors recently serviced.

New 255 fitted recently. Fuel pressure spot on.

Compression within a smidge of 150 psi on all cylinders.

Dizzy, leads and rotor have around 10000 miles on them.

Cleaned up earth by washer bottle and added additional earth strap to block. No change.

Issue with charging voltage, starts at 13.9 when cold and drops to 13.2 when hot, not sure if this is significant. Misfire is present from cold.

What should I take a look at next?
 

PobodY

Moderators
Staff member
Checked the throttle position sensor? - I think that at idle you're not using the lambda sensor for closed-loop, so I'd ignore that for now (I think that only comes into effect somewhere between idle and WOT).
 

Slunch

New Member
No I haven't checked the TPS, I'll take a look at it. Thanks George.

Lamda sensors pretty new, from Nissan, it's got about a 1000 miles on it. Before my down pipe sprang a leak AFR's were pretty well spot on according to my wideband. 14.7ish at idle and 12ish at WOT, dirty great lump of rich at lift off, pegged it most times.
 

Slunch

New Member
Any fault codes?
No fault codes when I last checked, it's worth checking again. And the TPS seemed to read alright on an Apexi Neo, 0-100%, removed and sold the Neo now, worth checking the TPS with a voltmeter though. I guess I'm going to end up on a vacuum leak hunt and swapping ignition components. No ones mentioned potential head problems which is a relief.
 

Slunch

New Member
Seemed to bleed OK. All pipes hot and hard, heater matrix hot, rad fans cutting in at around 90. I'll check coolant level tonight. Mentioned to a mechanic at work and he suggested running round the engine with a can of easy start looking for vacuum leaks. I ordered some cheap leads from flea bay just in case. Any other pointers gratefully received before I end up buying a new ignition system, couple of miles of vacuum hose and new IC pipe run. Lol
 

Slunch

New Member
Checked coolant, it took and egg cup full. Hosed down the engine with easy start, mildest hint of pick up around the IAC, I may have imagined it, sprayed a bit on the air filter and it ran beautifully. Did a fault code check, 55, checked lamda flashing on and off, but a bit erratic, responded well when I worked the pedal, showing rich and lean. Checked and set fuel pressure again, I do this with my thumb over the vacuum hose. Cleaned and repositioned earths on plenum, for good measure. I'll check TPS tomorrow, I have to remove some pipes to get to it.

I'm probably going to do the dizzy cap and rotor, anyone think it's worth sticking a new coil and igniter in at the same time.
 

Slunch

New Member
Sorted. After a good hunt, thankfully before I yanked the plenum off, I found a worn hose on the PCV system. Bunged up the valve and feed pipes and all is pretty good. Will fit dizzy cap and rotor anyway. Fair bit of smoke clearing crap out of the engine, and idle is now fairly steady around 900. Dump valve is next on the list.
 
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