79 928 backfired and stopped running

[Eric Drewes]

My 79 928 has been running great, then yesterday it backfired once and just quit. Could not get it started again. I towed it back to the house and started troubleshooting. I have verified that I am getting spark to the plugs and I disconnected one of the injector lines and I am getting fuel delivery to the injectors. When I crank on it, it almost starts, but just sputters for a few secs and then quits. I can't imagine it being anything with the timing, but I'm stuck. Any idea on what my next troubleshooting step should be?

[JK McDonald]

Hey Eric, If you have a good spark and fuel pressure, it may be that the timing belt has jumped a tooth or two. 1st make sure it is not something silly like a stuck fuel gauge.

A buddy had a similar thing happen when he quickly decelerated off the highway onto a side road to dodge a suspected patrolman. He had been hop scotching traffic with a Vette . His timing belt tension had evidently backed off enough for the cams to over spin the crank a bit which put everything out of synch. The engine would spit and sputter but would not start - after running perfectly only a few moments before.

Good Luck, Michael

[MPDano]

Rjm65 just had something like this happen a few days ago and he blew up his Fuel Distributor. On a 79 US too.

[Eric Drewes]

I did pull the timing cover to check the belt during my initial troubleshooting and noticed that there is a groove cut into the belt...my suspicion is that my tensioner is wore out pretty bad or out of line or something. The belt still feels tight and I didn't see any missing teeth, but either way when I saw that I knew that I'm overdue to replace the timing belt and will proceed with doing that. Hopefully when I get the belt replaced and everything timed up, it'll fire right up. It's my understanding that these older 928's are non-interference engines so there shouldn't be any valve damage.

[JK McDonald]

Hey Eric, You are right about the Pre-85 16 valve engine being non-interference. Where is the groove you mentioned on the belt - Back side/edge/center/etc...

It is virtually impossible to "feel" that a timing belt is tensioned correctly because the 928 belt is so long. The best test would be to inspect your timing marks or a quick check is to measure your compression on a couple of cylinders.

Michael

[mproseusa]

Hey Eric,
I always try to look for the simple solution. I had a similar experience with a backfire. This resulted in a "No Start" condition. I checked timing and quite a few other things that didn't help. I took a long look below the air manifold box (air distributer) and found a gap between the rubber connector and the throttle body. This of course resulted in the air intake sensor not operating. The air flapper could not open due to the massive air leak. The fix was to remove the air manifold box, re install the rubber connector and re set the air manifold box. Take a look under the air box, you might be surprised as I was. I'm not sure, but perhaps the backfire caused enough pressure to dislodge a "loose connection. In any case once the connection was corrected, the engine started right up.
Mike

[Eric Drewes]

Guys...I appreciate all the advice. The timing belt groove is at the front of the timing belt and has basically shredded appx 1/8 in off the front of the belt. I already had the airbox off too and the seal looks good and the flapper is not restricted. At this point, I'm leaning towards the timing belt having backed off a tooth or two. When the problem happened, I was doing about 35 in 3rd gear and had just let off the gas and dropped into 2nd gear when the backfire occurred.

[Eric Drewes]

One other question related to replacing the timing belt. I have another 928 that I bought as a project off a guy who had lost interest. He had purchased a bunch of parts for it several years back and never did anything with them. In one of the boxes of parts, there are 2 brand new timing belts....why he had 2 I do not know. My question is should I be wary of using one of these since they are at this point probably 5 yrs old even though they are still new in the box and were never installed? They look and feel good, but just wondering if these things deteriorate with age. A new belt is only $20-$30, but if I can safely use one of these belts I'd like to save my $. On the other hand I'd hate to put on one of these belts and have it go out on my 6 months later because the rubber separated.

[Pete R]

The early square tooth belts are a bit easier to jump. Age does effect the belt but probably much less so when not exposed to heat and fluids as it would in the car. it would probably be OK, I'd still buy a new one. If your motor is definitely non interference there would be very limited risk using it. What's more important is finding out what is causing the grove. My guess is the bushings in the tensioner arm are worn causing the idler pulley to cock.

[Roy928tt]

Have a quick look at the insulation block between the cold start injector and the plenum chamber after 30 years they deteriate and break up causing a lean mixture and poor starting.

[Mrmerlin]

take the timing covers off and replace all of the parts on the belt run, something serious is amiss

[DPW928]

Same thing happened on my 78 5 speed when the drivers side cam broke. That's the worst possible scenario. Did the backfire go through the intake or exhaust? If through the intake, look for blown vacuum hoses including the 8 connecting the spider to the upper plenum. If it backfired through the exhaust, look for something that would cause an over rich A/F ratio such as low control pressure or high system pressure. If the timing belt is correct with the crank at TDC and the notches on the belt line up with the pointers on the cam backing plate, turn the engine over and watch for a wobble on the cam gear. This would indicate a broken cam.

Dennis

[Eric Drewes]

Definitely jumped timing. Although I've got a stubborn bolt preventing me from removing the power steering pump bracket and hence the lower part of the timing cover which is preventing me from seeing the TDC mark on crankshaft, I have confirmed that both the cams don't line up evenly on the marks at the same time. I would think that both cams should be lined up at some point during the revolution and when turning by hand they never line up simultaneously. The culprit for shredding the belt is that top center roller, or at least initial appearances indicate that. I'll be having lots of fun after work for the next several days. Wish me luck, and thanks for all the advice.

Eric

[Pete R]

You'll know better once you get the center cover off. The center roller you are referring too is your water pump. It's obviously tracking too forward for some reason. keep posting pics

[JhwShark]

Bummer...You may want to consider replacing the water pump while you are there...

Jon

[Danglerb]

Dunno about replacing the water pump, if I could I think I would be putting it off even if it was for as little as 6 months when I suspect better more reliable water pumps will be known.

Maybe turn it by hand while the belt is off and kind of "feel" the bearing, and if its tight, leave it alone for now.

[Eric Drewes]

Here's my progress report with more pics. I've got past the stubborn bolts on power steering pump bracket and pulled the lower timing cover off. From what I see, the belt has been rubbing against the belt tensioner arm as well as the harmonic balancer. I turned the crankshaft a couple of times and there doesn't appear to be a warping of the harmonic balancer and I'm kind of puzzled as to why the belt has been tracking so far forward. The groove in the belt appears to have been cut in by the water pump roller as this is the only thing that has a raised edge on it. Could this have been caused by the belt being improperly positioned when it was last changed or was there maybe a spacer that wasn't installed on the tensioner? Definitely looking for suggestions here. This brings me to another question. Where exactly is TDC in the sequence of numbers shown below. I would think it to be the half hash between 1|0 . 0|1, but just want to make sure. I'm using a . for the half-sized hash mark.


^
2|0 . 1|0 . 0|1 . 2|0 . 3|0 . 4|0

[Eric Drewes]

Another note from the top picture in the post above...the belt is not making any contact with that roller at all. I can spin it freely by hand.

[Pete R]

It looks like your on TDC. It should be the mark between the Oand T. Well, the crank orientation is right anyhow. the cam marks have to be aligned too to be on TDC. Make sure you plug the hole to your oil pan if you take off your water pump. Coolant will run right in there.

[Pete R]

oh, your okay with the belt not running on the roller. It touches it when it's running and vibrating