1997 850 NA manual transmission vibration at idle (also sorting out NA EVAP hose routing)

Post by **abscate** » 24 Dec 2019, 03:58

I think you have really exposed new knowledge here, that the rear mounted FPR system uses canister vacuum to activate it. That makes sense from a materials standpoint is that it saves having to run a separate vacuum line to theFPR all the way to the back of the car

Imagine this, those wicked smart Volvo engineers write code into ECU that when the throttle is closed at high intakemanifild vacuum, it opens the purge valve to provide fuel pressure regulation

Later on, the FPR moves up front to either the fuel rail or the steering rack and gets its own dedicated intake manifold vacuum feed. This already happens on turbo models because you have to compare vacuum to snorkel pressure on those.

I think this makes sense?

v-janitor · Post by **v-janitor** » 24 Dec 2019, 14:39

Yes, I agree that that’s probably what’s going on. As a 25 year veteran of the IT Zoo, I can confirm that “If it can be done in software, it gets done in software”.

My replacement purge valve is supposed to arrive on Thursday, and when it does, I’ll measure the resistance (recall: old one is 25 ohms, and no longer reliably opens), and report back if this resolves the idle issue. I sure hope that it does, because if not I’ll then have to take a deep dive on fuel trims, etc. to convince myself that it’s the MAF or TPS...

Merry Christmas!

Post by **abscate** » 24 Dec 2019, 18:48

I’ve always wondered what the change was that got rid of the FPR in 1999

Those cars run straight 52psi at all loads.

They might have more bits of control in the injection timing and/or a faster clock speed on the CPUa

v-janitor · Post by **v-janitor** » 27 Dec 2019, 17:44

The new purge valve arrived today, and it has the same resistance as the old one, but unlike the old one, operates properly when energized. I installed it, thoroughly cleaned the spark plugs, and drained the power and did the relearn procedure on the ECU.

The vibration at idle and shudder at shutdown is still present.

So while the purge valve was bad, and replacing it was a good thing to do, it looks like it wasn't the dominant force causing the symptoms.

Now equipped with an ELM327, I looked at the short-term fuel trim, and it seems to be running more negative than positive (i.e. lowest value observed was about -8%, with highest about 6%) -- but these are not extreme values. I need to do some driving to get a long-term fuel trim number that I can trust.

Interestingly, I'm only able to see one of the O2 sensors (bank 1, sensor 1, which I believe is the front sensor) -- not sure if that's because this is an early implementation of OBD2, or something else. If bank 1 sensor 2 were not visible to the ECU, I imagine that I'd have a code and likely a CEL?

So, now I'm thinking about the affordable TPS ($28) and expensive MAF ($123). The MAF reading via the ELM327 seems to be working and shows about 0.5 lb/min at idle. Recall that the MAF was recently cleaned.

What should be my next move?

Thanks!

Post by **abscate** » 27 Dec 2019, 19:18

Check short term fuel trim at idle and at 2500 rpm

You are looking to see if the trim comes back to near zero at 2500, a sensitive diagnosticfor intake leaks

v-janitor · Post by **v-janitor** » 27 Dec 2019, 20:43

Here's the STFT at idle:

: STFT-idle.jpg (40.02 KiB) Viewed 1149 times

And here's the STFT at 2,500 rpm:

: STFT-2500.jpg (36.14 KiB) Viewed 1149 times

X axis is time (samples, I think). Y axis is in FT in percentage.

Both look "good" to the untrained eye.

Post by **abscate** » 27 Dec 2019, 20:48

Good fuel trims , no evidence of vacuum leaks.

I’ve never seen a TPS fixanything here.

Have you run the car with the MAF unplugged? Switch off, unplug, start up. You will get a mAF code set , of course.

That will run off the default software MAF. I’m doubting it will solve the problem but worth a try, and would rule out a pricy MAF

v-janitor · Post by **v-janitor** » 27 Dec 2019, 20:56

OK, thanks for the confirmation that I know just enough to be dangerous.

So what would be the recommendation for my next move?

Thanks!

Post by **abscate** » 28 Dec 2019, 05:17

Are you using poly for any of the motor mounts? They look great but transmit a lot of vibration.

I’ve never seen a TPS fixanything here.

Have you run the car with the MAF unplugged? Switch off, unplug, start up. You will get a mAF code set , of course.

That will run off the default air fuel map in the ECM. I’m doubting it will solve the problem but worth a try, and would rule out a pricy MAF
Running- wise, I’m stumped

v-janitor · Post by **v-janitor** » 28 Dec 2019, 17:25

All the motor mounts and torque bushings are OE rubber style (all replaced about a year ago).

I unplugged the MAF, and of course immediately got a CEL and a MAF code, but the idle was about the same.

I took some graphs driving (at normal engine temperature, 4th gear about 55 MPH, and 2,500 RPM) of the STFT and LTFT today; here's what they look like (left axis is STFT, right axis is LTFT):

: STFT-LTFT.jpg (45.08 KiB) Viewed 1136 times

What I find surprising is that even though the STFT oscillates nearly around 0%, the LTFT is around -6%. I was under the impression that LTFT is essentially an average of the STFT over time, so something doesn't seem right about the LTFT.

Could it be that this is a bad FPR with high fuel pressure (we know that the vacuum control side of the FPR is now fine)?

1997 850 NA manual transmission vibration at idle (also sorting out NA EVAP hose routing)

Re: 1997 850 NA manual transmission vibration at idle (also sorting out NA EVAP hose routing)