Misfiring and transmission shift problem - related?

gvanzant1 · Post by **gvanzant1** » 27 Mar 2013, 09:29

I have a 1998 S70, 2.4L, non-turbo that has been misfiring, sputtering and even stalling. It also has an issue where the tranny shifts into 4th(?) gear at low speeds (40mph) causing me to accelerate at low rpms. It will downshift into 3rd gear at 45 or if I accelerate hard or reduce speed below 40. But once it reaches 40, it shifts into 4th. It is during this low speed/low rpm mode that the car sputters most. It stalls when starting from a stop - not every time, but most likely when the car has been running for 10-15 minutes. The car idles fine - no revving or sputtering at idle. For the sputtering and stalling, I hooked up my code reader and it came back with "multiple cylinder misfire". I've replaced the fuel filter, cleaned the MAF sensor, replaced the distributor rotor and cap, removed (and discarded) the flame trap, checked the air filter (it's clean) and even ran premium gas for 3 tanks (I ususally use 87 octane). Performance improved after each for a short while but soon began misfiring again. This weekend, I'll replace the spark plugs (Bosch) and wires (Bougicord). But since the misfire is most noticeable when the tranny is in 4th at low speed/low rpm, I'm wondering if the tranny problem is causing the misfire or if the misfire is causing the tranny problem. If so, what would this be: a bad crankshaft sensor? Cam sensor? BTW, I keep the tranny in "Sport" mode.

Post by **cn90** » 27 Mar 2013, 09:35

Yes,

Fix the mis-firing problem first!

"Multiple misfires" = likely broken vacuum hoses. Get a flashlight and look around for broken vacuum hoses.

gvanzant1 · Post by **gvanzant1** » 27 Mar 2013, 10:06

Thanks. I'll check. Would broken vacuum hoses throw a vacuum leak code?

Post by **jblackburn** » 27 Mar 2013, 10:22

gvanzant1 wrote:Thanks. I'll check. Would broken vacuum hoses throw a vacuum leak code?

They'll throw O2 sensor codes and MAF codes and fuel trim codes.

Is your ABS light on? Is there a code for a speed sensor somewhere in there?

The transmissions in these cars are very good about input to throttle pressure (not like cars these days). It shouldn't be up-shifting to 4th that early unless it's confused about some kinda speed input.

gvanzant1 · Post by **gvanzant1** » 27 Mar 2013, 10:44

"No" to both ABS light and speed sensor codes. If it is a vacuum hose, where should I look first?

Post by **jblackburn** » 27 Mar 2013, 10:54

Hmmm...not sure on the weird trans behavior then.

Start at the end of the intake manifold tucked behind the power steering pump. There's a small vacuum port there for the PCV system. From there, investigate around the throttle body and turbocharger area.

gvanzant1 · Post by **gvanzant1** » 27 Mar 2013, 11:07

jblackburn wrote:From there, investigate around the throttle body and turbocharger area.

My S70 is NA (non-turbo) but I'll start with the intake manifold like you suggest. I replaced the PCV about a year ago so maybe something didn't get reconnected or broke during that repair.

Post by **cn90** » 27 Mar 2013, 11:28

Put in new sp plugs, cap.rotor (Bosch only) and the Bougicord.

Then look all over engine for any broken vacuum hoses.

At that age/mileage, chances are some hoses are already broken.

gvanzant1 · Post by **gvanzant1** » 02 Apr 2013, 21:12

Pulled the old plugs and found they were Joachim Platinum+4. Other posts I read suggested the single prong plugs, so that is what I used. Car has run fine since replacing those P+4s and the wires. The granny issue may be a temperamental "S/E/W" button. Overall, so far so good. I hope it lasts.