2002 XC70 misfire on cylinder 1--ECU issue?

[volvo245fan]

Hi there,
I have a 2002 V70 XC with just over 100k on it (new to me in the last few months)
I recently experienced some intermittent stumbling/misfire. I read the codes and found it wasn't getting a signal from coil 1 along with the expected misfiring codes. Been here before, so I assumed the ignition coil was getting tired, in my past experience once one coil goes, it's just a matter of time before the next one and the next after that, so I went ahead and put in 5 new coil packs and properly gapped plugs (all Bosch). The problem remained, so I next suspected perhaps the wiring to the cylinder 1 coil could be the culprit. I confirmed voltage and ground were good on coil 1 terminals 1 and 2 and traced the control signal wire from terminal 4 back to the ECU connector and confirmed continuity. I cleaned the ECU pins and terminal block with de-oxid and reseated the ECU, but problem is still there. Next, I went ahead and replaced the harness connector to the cylinder 1 coil, just in case there was an intermittent loose connection at the coil. The car usually runs perfectly when cold, but after 5-10 miles starts misfiring and CEL comes back (also flashes when the engine is under load)...the same set of codes returns every time...
Now I'm thinking it must be the ECU itself that's faulty (P/N 00627757A), but before cloning/swapping ECUs. is there anything I'm missing here? I assume if it were an injector or other fuel-related issue it would be throwing different codes and if it were a mechanical issue it would be more constant, not intermittent. The engine runs as smooth as any 2.4T I've ever owned when all 5 cylinders are firing.
Is there a definitive test to isolate whether the issue is in fact the ECU before I go this route?
Related, is there a reliable source for used ECUs that anyone would recommend? I'm wary of buying a random one off eBay...
Any help greatly appreciated!

[DavidE7]

Tough situation you have there. Some of these things below are things you probably tried and didn't add above...

Try testing the #1 cylinder compression to verify no mechanical faults preventing combustion. Try swapping the #1 coil to another cylinder just to make sure you didn't hit the 1 in 5 billion chance of getting a new Bosch coil that was bad from the factory and installed into the same position as the prior bad coil.

Try wiggling the harness with the coil covers off because cracked wire sheath can short with heat and vibration. Do you have any exposed ignition wires? Look very closely or do a check at night when you can see very small sparks. Wrap any cracked wire sheath with high temperature electrical tape and then enclose the harness with high temperature nylon split loom. Has the cover ever pinched the wires?

[volvo245fan]

Thanks for the speedy response @DavidE7--I did try swapping the cylinder 1 coil out after replacement, no luck there.
I don't have a compression tester so haven't gone that route, but I would think it would run kinda poor all the time if I'd lost compression on cylinder 1...
I've had the cover that hides the ignition coils off for weeks as I troubleshoot this. Wiggling the wire harness or the coil plugs doesn't help or hurt. Between the new connector for coil 1, and the fact that I have good continuity through the harness to the ECU even while poking around and nudging the wire harness, I'm pretty sure the wiring is good.
I'm not sure I follow your comment about exposed ignition wires--the business end of the ignition coils where the high voltage lives is hidden inside a rubber boot down in the spark plug wells--I've certainly seen the tiny sparks you mention and been rudely shocked by a few cracked plug wires on old distributor-based systems, but all the wiring here is low voltage. +12V, ground, and a 12V control signal that drops to ground to initiate a spark from the coil.

I haven't personally pinched the wire loom with the cover, but I've only owned this car for a tiny fraction of its 24 years...

[erikv11]

That's a puzzler.

I see you mentioned it but because it's reasonable to keep as long as possible the mantra "it's not the ECU, it's never the ECU," how about a problem with fuel supply to #1? Fuel injector issues give the same misfire codes, although should not throw a "missing signal" code for coils.

Check out the #1 injector as you did the coil (minimal effort to swap, also can look at the wiring, etc.).

Also as an exercise unplug the #1 injector and drive it around the block or even on the highway - you may be surprised how well it runs on just 4 cylinders, especially up above say 1500 rpm. When these engines burn valves, one option is to unplug the corresponding injector and drive it into the ground that way. Or drive it that way for a month util you can do the head work.

[jonesg]

Far as I know you can't swap in a used ecu, it has to be new and programmed to the car.
Its probably time to buy tools, a compression gauge is less than $30.
Eliminate the mechanical problems.

https://www.amazon.com/s?k=compression+ ... _sb_noss_1

[DavidE7]

The ground wires are exposed where they contact the block. I found that longer bolts can be used for a tighter attachment. I like to wire brush and put dielectric grease on the contact surfaces for the two ground bolts.

The Volvos in my fleet that had damaged split looms also had heat damaged wire insulation. I covered any damaged insulation with high temperature electrical tape and replaced the split loom protection to eliminated shorts. Loom replacement is very time consuming when done correctly.

I would test compression before trying to replace the ECU.

[erikv11]

... I like to wire brush and put dielectric grease on the contact surfaces for the two ground bolts. ...

Oops, bad practice, don't do this! TL/DR: Never use dielectric grease inside an electrical path. Use it outside or around the path.

Dielectric grease is an insulator.

Its basic purpose is to keep things, like moisture or corrosive chemicals, out of an electrical connection. By putting it on the mating surfaces you effectively interrupt the path to ground.

Obviously your engine grounds are working well enough so presumably you're squishing out enough dielectric grease that there is still a sufficient conductive path through there, but why sabotage the grounds!

Wire brushing or sanding the surfaces is productive, I'm with you there. The contact surfaces should be clean, flat and dry including free of rust or grease of any kind, then tightened down securely.

Using dielectric grease over/around a ground junction after everything is tightly fastened is good for preventing corrosion, but otherwise will not affect how well the ground conducts. And whenever the ground is removed you'll need to remember to completely clean off any grease or crap it attracted ("gunk"), so that no gunk gets into/onto the mating surfaces when you reassemble. I'd give it a miss altogether on engine grounds.

[volvo245fan]

Okay, I guess there's some consensus these symptoms could be caused by bad compression on cylinder 1, but would that really cause intermittent misfiring? I would think worn rings, stuck valves, blown head gasket etc. would manifest with a constant rough idle/misfire. The engine runs very smoothly until suddenly it doesn't and the CEL comes on. Could that really be due to poor compression? Also, would poor compression cause a "no signal coil 1" error code?
I bought a set of new injector o-rings, and pulled the fuel rail to investigate the effect of swapping injector 1 and 2 to see if that would shed any light, but the cylinder 1 injector had a substantial chip out of the plastic ring above the block-facing o-ring. That ring then came off completely while removing the old o-ring---it doesn't appear this plastic washer is a replaceable part, so I went ahead and ordered a new injector to put on cylinder 1.
I should have time to work on it this weekend and will report back what happens with the new injector and perhaps a compression test if that doesn't provide any insights...

[scot850]

Some auto parts stores still rent tools. You may be able to rent a compression tester. I believe the guys have already said what needs to be done for the first round of eliminations.

For me it is:

1) Compression check
2) Spark plug check
3) Coil pack swap with another cylinder to check if the issue is a bad coil pack.
4) Check coil wiring for damage or heat cracking
5) Check the injector wiring harness for damage

I have had issues where the code is not the actual cause, but eliminating these issues first are the cheaper simple tests.

Neil.

[vtl]

The car usually runs perfectly when cold, but after 5-10 miles starts misfiring

It is when the upstream oxygen sensor heats up and ECU switches from built-in air-fuel mixture software table to the closed loop. Replace the sensor. Also trace its wire down to the connector, it is usually covered in oil dripping from PTC nipple. Oil degrades insulation. I ended up splicing a new wire harness with connector and routing it up to the air box.

https://www.fcpeuro.com/Volvo-parts/V70 ... &year=2002