It was running fine before I replaced the head gasket, except for the head gasket being blown of course (do NOT use stop-leak in the cooling system on the straight 6 motor - the breather hose will clog at the coolant overflow reservoir, the system will get pressurized, and the head gasket will eventually blow).

So back to my problem - it ran fine before the head gasket being replaced, and now it starts up for a few seconds, then the idle drops and it backfires a few times and then unusually sputters out. Yes, I checked the timing. Then double-checked it. Then triple-checked it. Compression is good (around 150-160psi in all cylinders). She has spark. She has fuel delivery. I unplugged the MAF meter and no difference with it unplugged. I unplugged the cam position sensor and it won't start at all without that plugged in. I cleaned the EGR valve, and no difference. I sprayed carb cleaner into the air around the intake to check for a leak, and it didn't seem to affect the idle (although it's difficult to tell since it's idling so unpredictably and difficult to keep running for longer than a few seconds).

One thing I noticed when I had everything taken apart (before it was running rough) was that the throttle plate was very dirty. I sprayed lots of carb cleaner on that to clean it, and I'm thinking that maybe all the runoff from the cleaning contaminated the throttle sensor, which is now causing it to run rough. Or maybe there's an intake leak that I didn't find with the carb cleaner spray trick. Any ideas are much appreciated.

 

Sounds to me like you have an air leak, or missing spark to a cylinder.

Check your vacuum hoses, and the electrical connections to the coils, those wires are very brittle by now because of age. Inspect where the wire attaches to the connector, the wire can break and the insulation remains intact.

Can you locate what cylinder is missing? Maybe a spark plug picked up some debris and is getting shorted out? Are the coils on the correct cylinders?

I am betting air leak, look around the area of the clamp on the throttle body.

Maybe the throttle plate is not returning home where it should, the throttle cable is too tight?

DanR '94 964 358,000 miles (124,000 on the new engine)

 

Well, I checked for air leaks in the intake. I actually just disconnected the breather hose from the flame trap, connected a temporary hose to that nipple on the flame trap, disconnected the MAF meter at the inlet, clasped my hand over the MAF meter inlet to create an airtight seal in the intake, and blew into the temporary hose. It held pressure very well, so I don't think there's an air leak anywhere in the intake (or in the valves either, for that matter). I also checked the fuel pressure at the rail (unscrewed the small cap at the end of the rail and pressed on the schroeder valve pin at various times, including right when the car was dying at idle, and plenty of fuel sprayed out. So I think fuel pressure is fine. I can't imagine the injectors being fouled, since the car was running fine before I did the head gasket, and I didn't rub the injectors around in the dirt or anything. I checked for spark again also - turned the coil packs upside down with the spark plugs plugged into them and grounded, and watched the spark at each plug - seems to be sparking fine. So....with air, fuel and spark all ok, I'm thinking maybe it's the timing after all. The marks I made before disassembly are all lining up, but I'm trying to find some info on what the factory marks are and how to line them up. I have a '94 960 with slightly different plastic on the front cover, and it seems the "M" marks on the camshaft sprockets are in quite different places than they were on this '97 V90 (960) before disassembly. I don't really want to make the '97 camshaft sprocket positions match the '94, because I think they're so far off that there must have been a design change. Also, I'm getting around 155psi compression on all the cylinders, which I'm thinking wouldn't be possible if the timing were that far off (I could understand a tooth or two off). Any other ideas are very welcome (read desperation)

 

Some more notes that might be helpful...
- I noticed most of the spark plugs are quite fouled (nice layer of black on them) after only have the car started for a few seconds at a time, probably two dozen times. I don't know if that's an indication of anything.
- Also, I checked the voltage to the cam position sensor, following the Chilton manual. The numbers do not match what Chilton says they're supposed to be, and supposedly this indicates a problem with the electrical circuit. I'm not sure if maybe Chilton is wrong about that though, since I can't imagine how the circuit could have been damaged during the head gasket project.
- The engine starts fine and runs ok for a few seconds, then comes the glorious stumbling, backfiring, and stalling. So it seems to me like maybe mechanically everything is fine, but perhaps something electrical/sensor is wrong, and when the computer doesn't get the signals it's expecting everything falls apart. Just a theory.

 

Those m's on the cams are not the factory marks, the factory marks are very, very faint arrow heads, and they line up with the notches in the top cover of the timing belt housing.

I would check for the first, it still could be electrical, but you will spend much time down dead ends going that route.

Use something that will clean metal and buff up the cam gear surfaces to find those arrows.

I have included the a picture of the crank shaft mark on the back side of the gear, it is missing the serpentine pulley.

To check the cam sensor, just unplug it, it may sputter for a second but should recover, the computer will enter values on it own to keep the engine running, usually smoother than with a faulty cams sensor.

DanR

 

Got it. What should the crankshaft mark line up with? Or should it just point straight along the middle of the engine? Someone mentioned to me that he thought on this motor the crank is NOT supposed to be at TDC when the cam marks are lined up with the timing cover notches, but it's supposed to be a certain degree off from TDC (maybe 20 degrees or something). Right now I have the crank at TDC with the cam marks at the notches, so maybe that's the problem.

 

Your friend is correct, many interference engines do not use TDC for the crank, there is a line on the front side of the engine, this line is casted into the side, that is the mark for the crank pulley mark.

Once the crank is on it's mark the all piston will be far enough down the bores so the cams can be rotated without hitting(interfering) with any pistons.

DanR

 

Reservoir green Cap will burst if pressure threshold (IIRC 25 psi) is reached. I don't disagree with you on the the Bar's Leak though.

Compression is not good. Spec is 184-213 psi.

Have you done a leak-down test?

 

Here's the latest:
I checked the timing again, and it all lines up correctly. I swapped the fuel rail and injectors with another 960, and no change.

Something strange: I removed the oil cap, and when the motor is trying to run there are puffs of smoke coming out (presumably exhaust, smells like unburnt fuel plus exhaust). There's quite a bit. Does that mean bad valve seals so oil is leaking into the cylinders (would explain the fouled spark plugs) and exhaust is blowing up into the cam area? But what I don't understand is why it was running fine before I did the head gasket job.

Something else strange: the coolant temp sensor on the rear of the head has no connector plugged into it, and I can't find a connector for it anywhere. I'm not sure if it was ever connected, because I don't remember disconnecting it. I have a '94 960, and when I disconnect the temp sensor on that one, the motor struggles to start just like the '98 I'm working on. I'm not sure if the temp sensor is relevant to the problem or not.

 

Did you miss my previous post?

 

Yes, I missed that. I'll run a leak-down test.

 

I ran the leak-down, but first I did a compression check to have a baseline for comparison with the leak-down. All the cylinders were between 180 and 200psi except for cylinder 5, which was 160. I think before when I was reading them at 155 I was just not allowing the motor to crank for long enough. This time I gave it 6 or 7 cranks, and the pressure built up higher. I did the leak-down on cylinder 5 and it raised the pressure to 240psi. So it seems I have low compression on cylinder 5, but nothing that would cause the engine to stall. I think the engine should keep running on 5 cylinders because my other 960 will do that.

Here are my thoughts:
- Should the spark plugs be fouled like they are, all black? Attached is a picture of one. It's almost like oil is getting sucked into the intake somehow? Is that possible?
- I noticed all 6 or the coil packs have cracks in them, but my other 960 seems to be similar, so maybe that's not significant. A picture is attached.
- What would cause it to start up and run fine for one or two seconds, and then suddenly start stumbling and die? I checked the fuel pressure at the rail, and it's fine.

 

Never heard of 240 psi for leak-down!!! Regardless, where is Cylinder 5 leaking from?

Engine will not run for long with -1 cylinder.


From the photo, it looks like the spark plugs are carbon-fouled, not oil-fouled. Am I seeing them wrong?

If carbon-fouled, it means you're way too rich (too much fuel, not enough air). Since all 6 of them are fouled I would suspect:

1- Clogged Air filter or some sort of blockage on the air intake.
2- Insufficient ignition voltage.
3- Wrong spark plugs (I would get Volvo plugs, $ 30 for six-pack)

They're all known to crack. That's bad for the motor. They need to be replaced. People have found the cheapest ones on Rockauto.

What is your fuel pressure at the rail (in psi)? Have you pulled any codes? Do you have a real-time OBD2 Scanner so you can pull info on timing advance, etc?

 

I had this exact same issue after my head replacement on my 960. I tried the MAF and that didn't work. It ended up being a bad fuel rail. At first the leak wasn't bad but it got worse. After that was changed it runs fine.