Exhausting my knowledge, misfire issue on 1996 960 wagon.

[Bork Volvo]

I have a 1996 960 wagon with the B6304. Approximately 15k miles ago, I installed a factory reconditioned engine (block date code 1996) and did a bunch of "stage 0" repairs to bring it back to spec. This included PCV system, new fuel pump, engine grounds, etc. I also replaced the wiring harness to the coolant temp sensor, and installed a new electronic temp sensor since the old one was faulty. The car has new O2 sensors and a brand new exhaust and catalytic converter within the last 1,000 miles. The car ran tremendously until this summer.

In July, my 960 died inside of a car wash and developed an intermittent start issue that made no sense to me. Sometimes the car would start and run fine, other times it would not want to run at all.

Being the summer, I had other cars to focus on, so this project sat for approximately 6 months. When I was finally ready to work on this car, I used it as an opportunity to revisit my previous work and repair or replace some lingering issues, like my deteriorating coil pack wiring harness, and I replaced the fuel filter after draining the tank completely. I tested to make sure the voltage across the new harness was consistent.

I checked to confirm that my timing was correct, and inserted a borescope into each cylinder to examine the condition. All 6 cylinders looked nice and healthy, with no signs of detonation, oil consumption, or coolant leaks. Fuel pressure is ~44psi at the rail.

The root cause for my odd no-start condition was the crank position sensor (which was replaced at the time of the engine swap). With a new CPS, the engine started easily, but the car ran rough after a few seconds. I tested the amplifiers on either side of the intake manifold and verified their performance. I installed all new OE spark plugs and coil packs. I tested the fuel pump and regulator. I discovered that some of my original injectors stuck open after cleaning, so I replaced the injectors with a brand new set that was bench tested for flow spray pattern. Now the car runs much better, but it will throw a persistent random misfire code, and a specific code for 3 & 4 after extended idling or between 2k-3K RPM. I switched ECUs with another one I have from a 1996 wagon, and no improvement was made. Moving injectors, plugs, and coil packs does not seem to change the location of the P0303 and P0304 misfire codes, and the P0300 code persists.

MAF is clean, airbox flap door is operational, IAC valve is clean and functional.

When I did a compression test warm, the engine was within greenbook specs and appears healthy. At one point, I thought that I may have stuck lifter from sitting, but after an oil change and 60 miles of driving, I can't say it's any better or worse.

I'm just at a loss to see what else I could be missing here, but maybe the answer is obvious to somebody else. Please let me know if you have any ideas.

Thanks in advance!

[Offgrid]

Did you check the spark plug gaps when you fitted new plugs. The reason that I was able to buy my 740 for below scrap value, was because of a misfire. The owner had spent a chunk of money with a garage, and they had fitted new plugs, but they had assumed that the factory setting was correct. It wasn't - it was .2mm too wide, and that was enough to cause erratic running.

[Laird Scooby]

Another spark plug related question, what plugs have you fitted?

Also along the lines of your sticky lifter theiry, that's exactly what i was thinking too, i'd try adding between* a pt and a qt of Valvoline Maxlife ATF, NAPA MVA or similar ATF to the engine oil and run it for ~100 miles and then reset the codes.
Run it some more and see if they reappear, if they do inside 200 miles, reset again and drive again, monitoring to see if they appear.

To me it sounds as if, after a period of prolonged idling or gentle cruising, the times when the oil pressure is likely to ease back from the hydraulic lifters, is when the problem appears. The sticky lifter theory would concur with this and i've often had similar on one of my engines with hydraulic lifters after a period of lay-up.

That said, when i do an oil/filter change the first 0.5L (about a pt) of fluid into the crankcase is Carlube ATF-U which is a UK brand of similar ATF to what i've suggested for you. Then i top up with 10W40 semi-synthetic to level - helps reduce the risk of similar hesitancy/misfiring and improves economy, performance and emissions all round.

* I see you're in the USA so i've converted to real units for you instead of these nasty foreign units we have to use in the UK like Litres.

[940volvoman]

Yet another spark plug question....What is the correct gap,please?

[Laird Scooby]

Originally Posted by 940volvoman

Yet another spark plug question....What is the correct gap,please?

For your 940 Shaun, it is 0.7mm but i always say 0.65-0.7mm as most people use the method Haynes specify of a smooth sliding fit whereas it should be a tight sliding fit.

[Bork Volvo]

Originally Posted by 940volvoman

Yet another spark plug question....What is the correct gap,please?

I purchased the OE plugs from my Volvo dealer, part number 9146775 . They have three tips.

[Offgrid]

Cinderella is a 1988 740 saloon, and it has a replacement engine. I believe it is a na injected B230, which I was salvaged from a 1986 240, with an unknown mileage. It had new NGK plugs when I purchased the car, and they were set to a gap of .9mm. The cathode on the NGK plug is bifurcated, and the gap should be .7mm. .9mm seems to be the factory setting. I replaced them with Bosch plugs with a standard cathode set to .8mm, and that cured the erratic running problem.
The recommended gap for plugs seems to be .8mm, but bifurcated ( and presumably trifurcated ) cathode plugs need a smaller gap, but it seems that some mechanics don't understand this. I kept the NGK plugs, as I believe they may be superior to the Bosch, and I may refit them when I have had a chance to tune the engine.

I'm an enthusiastic amateur, and not a mechanic or an engineer, so if anyone can expand, or correct, any of my comments, I would be grateful.

[Laird Scooby]

Originally Posted by Offgrid

Cinderella is a 1988 740 saloon, and it has a replacement engine. I believe it is a na injected B230, which I was salvaged from a 1986 240, with an unknown mileage. It had new NGK plugs when I purchased the car, and they were set to a gap of .9mm. The cathode on the NGK plug is bifurcated, and the gap should be .7mm. .9mm seems to be the factory setting. I replaced them with Bosch plugs with a standard cathode set to .8mm, and that cured the erratic running problem.
The recommended gap for plugs seems to be .8mm, but bifurcated ( and presumably trifurcated ) cathode plugs need a smaller gap, but it seems that some mechanics don't understand this. I kept the NGK plugs, as I believe they may be superior to the Bosch, and I may refit them when I have had a chance to tune the engine.

I'm an enthusiastic amateur, and not a mechanic or an engineer, so if anyone can expand, or correct, any of my comments, I would be grateful.

Your belief of NGK being superior to Bosch is certainly correct IME, a single earth electrode (or cathode if you prefer, although that is generally used more for semiconductors and electroplating) is the correct variety for the redblock engines and no benefit is available from twin or triple earth electrodes, unlike the white blocks that are designed to have them.
As for the gap, all NGK plugs leave the factory with a 0.9mm gap as standard, except things like the BPR6E11 which has a 1.1mm gap - same as a BPR6ES but with a preset 1.1mm gap, i prefer to but BPR6ES and set the gap myself as it's a lot more wallet-friendly!

The red block is basically a very old engine design so needs a much smaller gap than many of the other cars of the 90s to run properly, hence 0.7mm being the correct setting. See my note above about normally saying 0.65mm-0.7mm for red blocks!

[griston64]

Originally Posted by 940volvoman

Yet another spark plug question....What is the correct gap,please?

For Redblock its 0.7 - 0.8

[baggy798]

Originally Posted by Laird Scooby

For your 940 Shaun, it is 0.7mm but i always say 0.65-0.7mm as most people use the method Haynes specify of a smooth sliding fit whereas it should be a tight sliding fit.

Either the 0.7mm feeler fits or it doesn't, it shouldn't be a tight fit where you're jamming it in.

Originally Posted by Laird Scooby

As for the gap, all NGK plugs leave the factory with a 0.9mm gap as standard...

Gonna need a source for that, can't see anything on google. Must be lucky as all the NGKs I've had have been 0.75-0.8mm out the packet.

Offgrid, it's all academic. Redblocks like standard NGK coppers (BPR6ES, BP6ES, or 1-2 heat rating colder, makes no odds), and you can slap them in without checking the gaps and it'll be fine. I did just that, and 3 years and 16,000 miles later they were still at 0.8mm.

And don't swig ATF whilst you're changing them!