87 745t, wiring harness new, temp sensor new, grounds are good, all basic tuneup stuff done. Random misfire at idle when the aux elec fan kicks on. It's not temp related, jumping the fan switch when cold produces same effect.

Thoughts?

Have you tried pulling individual plug wires when you replicate the miss trying to find the cylinder? If that doesn't work, maybe pull injector connector and see what happens.
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I've tried the pull plug wire or injector wire bit, the motor lays down so much I can't tell if the miss goes away. It's too subtle and the redblock is too twitchy and noisy when running normally it seems.

Have you tried the screwdriver against the injector to see if all are firing?
Or any plug that is dry or soaked with fuel?
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Plugs all look good, had them out a few times, and replaced them, also.

I put the injectors from my original motor and the replacement in, with new seals, and they were all pretty clean. It's vaguely possible but I see no evidence of a bad spray pattern. I could get a set cleaned and such, but that's getting expensive for guesswork.

Kinda out of ideas. Have you confirmed it is actually an ignition miss, or something else? I do know turbos absolutely hate anytype of air leak. Can you replicate with the fan disconnected and pushing gas at a higher idle?
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I haven't technically confirmed it's ignition, but it's my feeling.
Several of the intercooler/turbo hoses are new, at considerable expense I might add, thank you Volvo. I don't think there are any intake tract of vacuum leaks at this point. It's hard to say about the miss and higher idle, several parameters change with LH when the idle switch opens. It does away with the fan on ignition retard for one thing. It's definitely a more pronounced miss when that fan circuit engages though at idle.

Hey Rex,


Sorry to hear about your problem. What spark plugs are you using? How new is your oxygen sensor? Is your power stage firmly screwed to the fender with thermal compound? Is it original? I assume that the aux. fan is intended to turn on when the motor is hot. If the power stage is weak or getting too hot, perhaps it could cause a misfire?


Regards, Jon

Plugs are NGK or whatever stock is, clean, stock gap, nothing fancy.
The 02 sensor was new when I put the motor in, and I replaced it once in the course of trouble shooting, no change. The power stage is new and firmly attached so it stays cool. I had the power stage thought as well, but it's the same song and dance with any of the several spares I have, and the new one.
The coil is new as well.

Keep the ideas coming.

Hey Rex,


Have you tried swapping out the ignition computer? Perhaps its causing the misfire when it retards the timing.


Regards, Jon 'Uncle Olaf'

That is one of the few things I have not swapped out, I haven't run across a spare. It's worth a shot I suppose.

So..

Good thoughts. I've had most of them.

Let's try a few more hints.
I've had 25mpg or better the last 20K miles, which have been inside a year.
Oil usage is nil. No smoke. There is piston slap, bad, it's #2, compression was slightly low on that one, but "low" was still well within spec, and they were all within range of each other.
It does idle slightly rough, though I think that is partially a Volvo thing as I've seen a bunch that did. You can feel a little rock in the car now and then, irregular, subtle. Yes the motor mounts are new.

Strangeness, strangeness. I bet if this was LH2.4 it'd be covering up whatever it is.

Is it the original fan? The brushes could be/probably are worn alot and it may be drawing excessive current. But check the distributor for a worn wobbly rotor shaft. You could have a weak spark to begin with. Do you have Bouricord brand ignition wires?

Another couple good thoughts, but alas no.
The plug wires were originally cheap Bosch, which worked fine for the few months I had them on there. It was upgraded to 8.8mm Accel wires I put together from the universal V8 kit, there was no difference in function between the Bosch and the Accel. The distributor is a new Bosch re-man unit.
Also on the fan, it's a thought I had as well, electrical resistance, bad grounds, etc. So I have, for a 2nd time, awhile back cleaned and inspected all the electrical connections underhood, and all the grounds from the firewall under dash forward. And it isn't the fan itself, since one can unplug the fan, jump the temp switch that activates it, and observe the same random miss.
Neat huh?

I've also verified operation of the TPS, and idle control motor.
For what it's worth.

Weird problem

...since one can unplug the fan, jump the temp switch that activates it, and observe the same random miss....

1. Maybe you have a wiring harness fault.
Try unplugging the fan and pull fuse 7
Then measure the fan switch leads to ground.
check for High and low resistance

2. Maybe the Fan relay has a fault/ loose socket pin

3. Fuse 9 and 10 are also are involved

Post back.

Bill

That I will look into. Though the harness is the newer non-rotting version, and I've checked all the plugs and such for pushed out pins, cleaned the grounds, etc.
Can you tell I've owned old euro cars before? :)
I'll try pulling the fuse and such. I think it's the timing being retarded that it dislikes so much, just seems strange that it misses like that and runs so well otherwise with good MPG and such. I can snip the line that tells EZK to retard timing when the fan comes on, but I'd really like to know why.

The most likely culprit is a high resistance short in the wire from the fan
switch to the fan plug. It can't be a low resistance short or it would pop the fuse.

It's also possible that you need to service the positive junction
of three heavy red wires near the ECU. Something loose there maybe.

I'll check the continuity of the wiring, and I know I cleaned the ground block by the ECU, don't remember if there were hot wires there, but I'll look.

If the Bosch or Accel plug wires have the usual crimped connections on the distributor ends then they are probably not making direct contact with the posts on the bottom of the holes in the distributor cap. They may feel pushed in all the way but there's probably a gap between the crimp end and the tip of the post. This makes the voltage jump two gaps and weakens the spark. Bouricords are the ONLY way to go.

I thought about this looking at the cap, and while I can't say now how they looked I was content at the time with the way I was able to tweak the wire end on the accel set and install them. I could stick some bogicord's on there, another little step in a depressingly long process of elimination can't hurt. :)

Is it the original fan? The brushes could be/probably are worn alot and it may be drawing excessive current. But check the distributor for a worn wobbly rotor shaft. You could have a weak spark to begin with. Do you have Bouricord brand ignition wires?

Actual miss, or erratic idle? Air leak, throttle body/ICV dirty? Just some ideas.
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Nope, been there done that. It's actually related to the fan being switched on.
Actually it's related to LH2.2 retarding the timing like crazy when the fan switch is activated. But it shouldn't miss, and it does, random single misses, no pattern, can't track a particular cylinder. I've swapped ECU's as well.

Harmonic balancer separating?
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Good call, but no. When I put the motor in I manually checked TDC, and also marked the balancer so I could see if (when) it slips. So no, the timing isn't off. I wish it was.

Howdy,

I just read all the posts to this point. It seems like you have covered about everything. Have you moved the power source for the fan from where ever it is now to a new spot? For instance, if the power is coming from fuse x, move it to fuse y. I think the fan motor may be giving a little inductive kick onto the power wire. If all else fails, run a dedicated power wire from the battery post.

--sd

I had that thought too, I could tell you stories about old VW blower motors doing crazy electrical stuff. On this one, even with the fan unplugged, it acts up.

I too have a random miss at idle... no apparent cause. Even have had my valves adjusted and a lot, a LOT of maintenance parts replaced, but never the coil.

I have 264K miles now though, original motor *I think* and rebuilt once I know. It's LH 2.4, and never throws codes.
I'll be watching and waiting to see if I can contribute.

I went with kingsborne wires, and they are excellent, and do make the proper contacts.

What about your voltage regulator or an accessory with a bad bearing pack that occasionally grabs.
Maybe the extra load from the fan is kicking out more from the alternator and increasing resistance enough to make the bearings catch just a little, once in a while. You could disconnect the alt belt and run it without for a little bit to see it if disappears.

-Will
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1990 740 Turbo, on its way to stock specs, maybe beyond

Alternator's been swapped for a bad regulator awhile ago, same with or without.
I have seen an accy catch like that though.