1998 V70R: computer alleges misfire on 4 cylinders under full throttle at high passing speeds

atomicthumbs · Post by **atomicthumbs** » 22 Aug 2022, 13:17

I just got back from a long road trip, and discovered that around 80-90 mph and with the pedal to the floor, the check engine light starts flashing. I don't actually notice any roughness, stumbles, or drop in power when it does this.

Scanning the codes immediately afterwards gives the result in the attachment.

I suspect (but don't have the tools to confirm) that I might have a vacuum/boost leak; there is a hissing noise that gets louder with boost, and I'm not sure it's normal induction sound. I know I have an EVAP leak.

My top things to check so far are for a vacuum/boost leak, find the EVAP leak, and ignition system from the coil onward. It's an MSD coil from IPD, which I replaced 6 years/~60k miles ago; I've got no idea how long the MSD units last. PCV system was done 65k miles ago.

Is there anything else obvious I should adding to the list?

Post by **scot850** » 22 Aug 2022, 15:22

We have covered the vacuum leak issue many times. I would strongly suggest having the vacuum and EVAP system smoked as it is by far the quickest way to find leaks. You can spend hours looking for leaks and still miss it.

I don't expect the EVAP leak to cause your misfire though. If you have a PCV/emissions system blockage then maybe.

When were your plugs last replaced, checked and gapped? When was your plug wires last swapped out, as well as your distributor and rotor arm?

Do you still have your original coil? Otherwise a used/junkyard one is worth having to try.

Neil.

Post by **abscate** » 25 Aug 2022, 02:45

At those speeds you need genuine Bosch cap rotor and plugs bought from reputable places, like not eBay, Amazon, Stone Age auto , or the oh oh oh oh bad parts place.

Misfires are detected by watching crankshaft acceleration timed to each cylinder firing. It’s possible that misfires are not reported correctly especially if it’s multiple misfire codes.

I don’t know M4.4 well but it’s grown up brother ME7 uses a set of crankshaft adaption values to map each position of Tdc for each cylinder to guide in misfire codes. You can check these in VIDA or it’s m4.4 version ( vol fcr?)

Post by **Chuck W** » 25 Aug 2022, 04:02

abscate wrote: ↑25 Aug 2022, 02:45 I don’t know M4.4 well but it’s grown up brother ME7 uses a set of crankshaft adaption values to map each position of Tdc for each cylinder to guide in misfire codes. You can check these in VIDA or it’s m4.4 version ( vol fcr?)

The '98 is still the same ignition set up as the 850's, and not much more "smart" than the M4.3 in regards to detecting which cylinders are misfiring. It's a single coil set-up.

It could also be a fueling issue under those loads, especially seeing as you may have some sort of vacuum leak. (You never mentioned anything about the fuel system fitness) When was the fuel filter last replaced?

Sounds like it might be time for a "Stage 0" refresh.(It's been 6 years since it sounds like things have been gone through)

Post by **Sveedy** » 25 Aug 2022, 14:38

I believe that you're running lean when you see the flashing CEL under WOT. That causes knock, which sets off the CEL.
At least that's my understanding.

Post by **scot850** » 25 Aug 2022, 15:05

What fuel are you running in the car? Should be Premium gas 91/93/94. If you want burned valves WOT will do that for you!

Neil.

Oly850 · Post by **Oly850** » 01 Sep 2022, 16:34

I like what others had to say about checking your fuel pressure, when you’re going that fast, you’re using exponentially more fuel with much less time to open the valves and burn it.

- are you sure you want to go 90mph in this car? - - Do you have Z rated tires?

Anyway I’d also make sure your cylinder head ground straps are clean and in place as well as the connections to the distributor. +1 on throwing away your distributor cap and getting a genuine Bosch one.

Thomas