"Backfiring" when starter motor is running - Page 2

BGA · Jun 8th, 2011, 08:55

Yes. It was the position sensor. I picked one from a scrapyard (5 GBP) and now it runs perfect. Thank you all.

Bo Gunnar

Oilydad · Jun 8th, 2011, 10:28

Just as a completely different idea on this. You say it's not a fuelling problem....but:

My own 940 was giving very similar symptoms recently, and it turned out to be a worn out armature in the in-tank fuel pump.

Think of it like this. The end of the armature has about 12 segments. If 4 of them are BAD segments, if the motor stops on one of these when you turn off the ignition, then the car won't start the next time you want to use it. In that case no fuel will pump through. If it ends up on a GOOD segment, then the car will start, and the pump, through it's own spinning motion will keep running well enough to keep the pump running.

It will be EXTREMELY rough whilst attempting to start, juddering and backfiring etc if the pump won't run. It's not visible, and you won't hear it.

The simple test (courtesy of, and thanks to my own recovery Service) is to go under the car and give the bottom of the tank a good thump with a wooden or rubber mallet whilst another person turns the starter over as normal. This has the effect of jarring the armature enough to make the pump run and start.

It's only a thought.....but like I say, it's very similar to the symptoms that I was having, and like you, I had assumed that it wasn't a fuel problem because the fuel was always coming through when I tested it. Being intermittent, it was only happening every 10 or so starts.

IF it's this, then the only cure is a new pump, and I recently posted on how to do this very job.

BillB · Jun 8th, 2011, 23:28

My '89 745 (with hall sensor) does this occasionally whilst cranking from cold i.e. the engine almost comes to a stop with the starter barely able to turn it. It's as if the timing has suddenly advanced which is pretty unlikely. I think it tends to happen if it doesn't fire early and there's maybe a builld-up of unburnt mixture in the cyinders i.e. it's flooded. If I leave it a while it starts up and runs normally. Strange.

Tardis007 · Aug 30th, 2012, 00:24

I have a very similar issue with my B5204t engine Cranks Stops (Back fires or compression lock)and then normally starts 2nd or third attempt. I have found turning the key off and then trying again usually allows it to start. (Live data of the timing shows a fluctuation from 0deg to 20 deg at idle)I know this thread is getting old now, but would be interested to see if you ever sorted it?

Jun 8th, 2011, 10:28	#12
Oilydad Premier Member Last Online: Mar 21st, 2016 21:21 Join Date: Aug 2009 Location: Exeter Devon	Just as a completely different idea on this. You say it's not a fuelling problem....but: My own 940 was giving very similar symptoms recently, and it turned out to be a worn out armature in the in-tank fuel pump. Think of it like this. The end of the armature has about 12 segments. If 4 of them are BAD segments, if the motor stops on one of these when you turn off the ignition, then the car won't start the next time you want to use it. In that case no fuel will pump through. If it ends up on a GOOD segment, then the car will start, and the pump, through it's own spinning motion will keep running well enough to keep the pump running. It will be EXTREMELY rough whilst attempting to start, juddering and backfiring etc if the pump won't run. It's not visible, and you won't hear it. The simple test (courtesy of, and thanks to my own recovery Service) is to go under the car and give the bottom of the tank a good thump with a wooden or rubber mallet whilst another person turns the starter over as normal. This has the effect of jarring the armature enough to make the pump run and start. It's only a thought.....but like I say, it's very similar to the symptoms that I was having, and like you, I had assumed that it wasn't a fuel problem because the fuel was always coming through when I tested it. Being intermittent, it was only happening every 10 or so starts. IF it's this, then the only cure is a new pump, and I recently posted on how to do this very job. __________________ Are We Nearly There Yet! No longer a VOLVO owner, not by choice, but 'cause they don't make proper Volvos any more!

Jun 8th, 2011, 08:55	#11
BGA Junior Member Last Online: Oct 12th, 2021 10:18 Join Date: Dec 2008 Location: Kagerod, Sweden	Yes. It was the position sensor. I picked one from a scrapyard (5 GBP) and now it runs perfect. Thank you all. Bo Gunnar

Jun 8th, 2011, 23:28	#13
BillB VOC Member Last Online: Apr 24th, 2024 13:58 Join Date: Dec 2002 Location: Bristol	My '89 745 (with hall sensor) does this occasionally whilst cranking from cold i.e. the engine almost comes to a stop with the starter barely able to turn it. It's as if the timing has suddenly advanced which is pretty unlikely. I think it tends to happen if it doesn't fire early and there's maybe a builld-up of unburnt mixture in the cyinders i.e. it's flooded. If I leave it a while it starts up and runs normally. Strange.

Aug 30th, 2012, 00:24	#14
Tardis007 Junior Member Last Online: Mar 3rd, 2013 03:57 Join Date: Jul 2012 Location: Auckland	I have a very similar issue with my B5204t engine Cranks Stops (Back fires or compression lock)and then normally starts 2nd or third attempt. I have found turning the key off and then trying again usually allows it to start. (Live data of the timing shows a fluctuation from 0deg to 20 deg at idle)I know this thread is getting old now, but would be interested to see if you ever sorted it?

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