Strange cut out and backfiring, no start

cj.surr

Well-known member
Joined
Jan 27, 2017
Location
Upstate NY
TDI
AHU Volvo 240
This engine is an AHU that has been put in a Volvo 240. The car had no problems starting and running for the first few hundred miles. Then, I would sometimes have a hard starting issue. Either it would take several seconds to start or would start and run really rough <500 RPM. If it was running rough, I would usually turn it off and upon restart it would run normal. These conditions would happen when the engine was warm or cold, but seemed to occur more often when cold.

Today the car would not start at all. Several long cranking sessions would not get it to start, although it would sometimes fire. My first guess was that there was air in the lines. I don't have any clear fuel lines, so I filled the feed line and fuel filter. I also tried to use a vacuum pump to pull fuel from the pump return fitting, but it was unable to pull much fuel at any rate. After hours of attempted starting after bleeding was unsuccessful, I started it by pulling it in 3rd gear with a truck. Once it started, it ran normal. I drove it around for a few miles but during that drive another issue started,

The engine would run fine shifting through the gears, but if you let off the throttle with it in gear, and it went into deceleration fuel cut (0% load on the OBD2 software), returning to throttle would be a mess. Returning from fuel cut, load could be at 50% but the engine was still decelerating. This could last for seconds, and smoke would come out of the tailpipe during this (I even heard it backfire through the exhaust a few times which was very surprising). Eventually it might bang back to life and run normally, but it would start all over again if I let off the throttle. If I were driving in gear and toed the throttle down to 1%, and it did not enter fuel cut, it would run completely fine, full power. Also, if I went from a fuel cut deceleration to idle (push the clutch in) it would run fine also, never encounter the hesitation. The hesitation would only occur if I tried to return from deceleration fuel cut out at higher RPM (2000+).

Once I turned the car off, it went back to no-starting. I have a clear fuel line in there and I can see that it is primed. I am not sure about the injector lines themselves as I haven't tried to bleed them (since it's not running). However, this seems to be an electrical issue to me, because of how systematic it is.

The only relating code I got was the "N108 short or open to ground". I think this was because I had unplugged it at one point with the ignition on while I was trying to bleed. regardless, I tested it. With KOEO, 12v on the harness side, 15ohm on the sensor side. So it seems to be fine.

Also, I checked the timing of the camshaft and fuel pump, and they are both in line.

I hope this explanation makes sense. It seems like a very strange issue to me but I am new to electronically controlled diesel. Hopefully someone can suggest another area to check.

Thanks in advance.
 
Last edited:

jhax

Veteran Member
Joined
Jan 24, 2012
Location
Golden, CO
TDI
96 Passat B4V, ALH engine out of a 2002 Jetta, some IE Rods and ASV Pistons. Nothing drivable yet though
MAF sensor?
 

cj.surr

Well-known member
Joined
Jan 27, 2017
Location
Upstate NY
TDI
AHU Volvo 240
The car would not start today again unless I pull started it.

Recap of issue,

-Driving along, let off throttle in gear, enters deceleration fuel cut
-return throttle, car does not accelerate, load % increases, smokes heavily, backfires, (timing says 63.5 deg advanced on OBD2 app while occuring)
-push clutch in and allow to return to idle, everything is back to normal

When not experiencing this condition, the car has full power. It will often hesitate when shifting gears (and puff smoke). It idles fine, too. Also will throw a little black smoke under heavy accel which it did not do before.

I am thinking that maybe the computer is entering some kind of mode to dump fuel into the exhaust to heat up the cat or something? And it's just occurring at the wrong time and for too long?

Thanks
CJ
 
Last edited:

ketchupshirt88

Veteran Member
Joined
Oct 31, 2012
Location
waupaca, WI
TDI
2005 Passat daily, a bunch of others in the graveyard out back...
I am thinking that maybe the computer is entering some kind of mode to dump fuel into the exhaust to heat up the cat or something? And it's just occurring at the wrong time and for too long?

CJ
Does it even have temp sensors for the cat? i thought was just on the 1996 with BK ecu and 5th injector. I do not think this is your issue.

some of those symptoms sound like when my EGR block off plate had warped and the gasket had blown out. causes turbo to not spool properly and makes her smoke. look for intake and exhaust manifold/EGR leaks.

my idea wouldnt cause a no start issue though... makes me think electronic issue as well.

the "backfire" issue beats me though, ive never experienced that.
 

cj.surr

Well-known member
Joined
Jan 27, 2017
Location
Upstate NY
TDI
AHU Volvo 240
I hooked the car up to a vagcom. It is still completely unable to start cold. It does not seem to get a steady reading of injection timing when starting.

Once the engine warms up, it will start with a lot of effort and cranking. Requested timing is about 11deg BTDC (which seems high?). Valve at about 40% DC.


During idle and cruise, requested and actual fuel timing is basically spot on, with N108 duty cycles in the 80-90% range.

Again, the car drives completely normal until deceleration fuel cut is reached. Upon reapplying the throttle I get (along with a lot of smoke and no acceleration):

Requested timing 11deg BTDC
Actual timing 28 deg BTDC
N108 valve 2%

It seems like the N108 is getting the wrong message, but I'm not sure if it could vary the timing that much (~17 deg?).


Any help/insight would be greatly appreciated.
 

cj.surr

Well-known member
Joined
Jan 27, 2017
Location
Upstate NY
TDI
AHU Volvo 240
Figured I should update this thread because I never did and it had such weird symptoms. The problem was actually caused by a loose crank bolt. As a result, the D-shape on the crank and timing cog had worn out significantly and was affecting timing. I assume that's why it started acting up when switching from accel to decel.
 

cj.surr

Well-known member
Joined
Jan 27, 2017
Location
Upstate NY
TDI
AHU Volvo 240
I don't think I've had any related problems in the last 60k. The crankshaft D-shape was excessively worn and I had to drill/dowel a new sprocket on so that it would stay in alignment. It ended up being 1-2 degrees off according to the mark on the plastic timing cover (it's a swap, so no flywheel mark to go off of).
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4 (sold), '97 Jetta (scrapped)
I read your thread in the conversion forum. I’m curious what the issue ends up being...

-Todd
 
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