Ocassional bizarre RPM spike while starting?

[ketchup3821]

Hello.

I have a 2001 V70 (LPT) with 157k on it, owned since 140k.

Maybe about 30% of the time, the car will take slightly longer to crank and start, with a bizarre very quick spike up to around 2-3k RPM before starting (and then going up in RPM as one would expect once it starts). The car does always start. Some vague internet research (non-Volvo-specific) says it this could be the crank or cam sensors failing?

This first happened once right after I changed out the ETM (at 146k miles), but didn't recur until a few weeks ago (when it got colder, possibly related; I live in the Chicago area). It idles fine and performance/everything else seems normal. It starts a little slower in the cold (-3F this morning), but as one would expect.

I did the spark plugs (Volvo brand) last at 146k and also replaced one ignition coil (Autozone brand) but the rest are original as far as I know. Also replaced the battery a couple months before that.

In addition, I've gotten a P0422 "Main Catalyst Efficiency Below Threshold" code twice now, months apart (didn't recur either time). Not sure if that's related.

Any idea what's going on? I want to fix this soon in case it eventually leads to something stupid like a no-start. Thanks.

[mrbrian200]

Cam/crank sensors are a 'maybe' at best.
To clarify, the tach jumps to 2-3k while cranking, but returns to normal as soon as the engine gets running? Correct?
If you leave the key in the KPII/run position without cranking the starter/starting the engine the tach stays at 0 rpm?

If both of those conditions are true, I would be more inclined to suspect an electrical grounding issue: Engine to subframe, main battery grounds cables, the braided ground strap to the strut tower, or (possibly) one of the ground wires that bolt to the fender. The starter draws current in fast short bursts as it turns (it's a DC motor with brushes). Somehow a current is being induced through crank sensor circuit when the starter is engaged that the ECU is interpreting as sensor output. As the current drawn by the starter viewed on an oscilloscope would be a fast square wave, this I conclude is what the ECU/tach is seeing as 2000-3000rpm.
Its also possible a starter winding is going bad either spiking the electrical system or producing so much EM that the crank sensor (nearby) is picking it up. However, consider that a plausible but unlikely 'theory'.
Also possible: for a short to ground in the sensor wiring + bad main engine or chassis ground(s)

[ketchup3821]

Thanks. It's rather cold here right now (Chicago area, -13F this morning; yes my Volvo started) but when it warms up a little I'll take a look at some of the grounds.

[zaneallen]

mine used to do this when the engine coolant temperature sensor was bad

[zanzabar]

viewtopic.php?f=9&t=73914

I've had this same issue for well over a year now. I've thrown lots of parts at it. Checked grounds and all look good (granted, looks can be deceiving).

I'm leaning towards mbrian200's suspicion of the starter. That might be the next thing I throw at it.

[ketchup3821]

Thanks zaneallen and zanzibar for your input too. After I check the grounds, I'll consider the ECT sensor (had one go bad on my previous car so that one's top-of-mind) and/or the starter.

[dcm0123]

Had a similar problem on a 2001 S80. Was not able to fix but will tell you what I did.

Ran additional heavy gauge copper ground connections between transmission and engine as well as between the starter attachment bolt and the inner fender well (chassis).

Read once that if the transmission is replaced (as in my case) sometimes you need to clean the contact surfaces between then to prevent problems like this. This is why I added the external ground between the two.

The grounds were made from #2 AWG copper wire- battery cable with crimped lugs on the end.

Starter or sensors could be the cause. Let us know how you make out.

[mrbrian200]

Capture.JPG (10.98 KiB) Viewed 2880 times

Another educated guess/theory:
Metal ring the hold down bolt goes through might function as a chassis or 'earth ground' connected to the metal shield around the sensor to give it a directional pickup pattern. If this ground is lost the sensor would become more sensitive to spurious EM pulses such as that produced by the starter motor. I think that is what people were getting at by addressing the electrical continuity between the transmission case and the engine block. The break could be between the sensor shield and the metal ring (internal). This sensor does not get any sort of ground through the wiring harness... 2 wires: reference voltage and signal return only. Being a hall effect sensor it is potentially sensitive to EM. The million dollar question being what exactly is it picking up, and why, seeing as it is designed to normally reject this interference. Otherwise the tach would jump on all our cars.

Edit: there's another possibility. The ECU likely has filter of sorts that normally rejects spurious/low intensity EM from being counted as lobes passing the front of the sensor. You might check with xemodex to see if this is a potential failure point on ECUs they've ever encountered.

[mrbrian200]

Another member noted a similar problem, the issue eventually progressed to seeing DTCs for misfires on cyl's 1+2.
It may seem like somewhat of a long shot to clean the two ring terminals that ground the coil packs (underneath the plastic cover next to the coil packs). As cleaning these coil pack ground rings doesn't cost anything to do yourself, doesn't hurt to try.

Read my post on page 2:
viewtopic.php?f=9&t=73914

[zanzabar]

Another educated guess/theory:
Metal ring the hold down bolt goes through might function as a chassis or 'earth ground' connected to the metal shield around the sensor to give it a directional pickup pattern. If this ground is lost the sensor would become more sensitive to spurious EM pulses such as that produced by the starter motor.

I'm going to take a look at the sensor again and clean off the surfaces.
Thanks again for the input mbrian200!