98 v70 intermittent big misfire

[songzunhuang]

I had this issue for years before it was tracked it down to a bad manifold gasket. A small piece on the underside had blown out and there was no way to see it from above (still difficult to see from below for that matter).

Like you, I was throwing parts at it. The good news is that much of my fuel and ignition system is new now!
Good luck to you.

[kmashlan]

OK - still no luck. I think I may have to take it to a mechanic - I hate it when I have to throw in the towel!

Things I have now done:

New Plugs
New Rotor and Distributor
New Cam Position sensor
New fuel pump relay
new injectors
new PCV system - Vacuum pipes and tank
new Inlet manifold gasket (part of the PCV kit - nice kit from FCP!)
New battery
Swapped MAF.
new coil
Compression is 115 across all cylinders (when cold)
No ticking from the spark plug leads (have swapped with another set and no difference) but have not put new leads on it. Including the coil lead.

Issue is intermittent. I assume anything major mechanical (compression, valves etc) would not come and go. No codes on the computer.

Only things I have left are:
Fuel pump - does not change pitch or stutter when the car is running badly - doesn't mean it isn't dead though. I don't have a fuel pressure gauge.

Of interest after changing the PCV system, the dipstick still has smoke coming from it - maybe something is still blocked somewhere? The passages the PCV tank attaches to seemed pretty clean. Although would this cause an intermittent issue? Of note, the car seems to run well when really cold - it might start misbehaving when the car has warmed up fully. Weird behavior - took it to work and it ran perfectly. Left it in the carpark for 8 hours, started it and it ran badly right from the get go.

I have access to software for the car, so is there anything I might be able to learn from diagnostics?

Cheers.

Ken

[Clemens]

I´m thinking maybe the coolant temp sensor. This could cause a rich or lean condition because the ECU gets the wrong themperature reading. If you replace it, only get the OEM part, the cehap one on my 95NA doesn´t work properly.

Have you checked all ground cables and the big +12V one?

As for the PCV: Do you get vacum with the glove test? I believe to remember that the smoking dipstick is not very accurate on these cars, meaning it can still smoke though there is sufficient vacum.

[erikv11]

Tough one.

I'd throw a new Bosch fuel filter at it, if that hasn't been done recently.

What is the date code on the ignition wires?

It didn't pop a rear cam seal did it? The one under the distributor cap. Also, does the cat still get red or was that a one-time thing? I do agree a clogged cat wouldn't be so intermittent, mostly wondering if it always dumps fuel/runs rich.

[sleddriver]

When's the last time the throttle body was cleaned?
How about the IASC (idle air speed control) motor?
Check for vacuum leaks at the intake vacuum tree o-ring, and all of the associated vacuum hoses found there. If the tree caps are hard-as-a-rock, they won't properly seal.
Smoke both the intake & motor before throwing any more parts at it.

[kmashlan]

Cheers for the suggestions guys (and gals...)

I will try these out and let you know.