1995 850 Spark,fuel,compression and no start

[momentumbob]

This is a Hail Mary because if somebody described this issue to me I would say they didn't know what they were talking about. 1995 850 wagon starts easily and runs well from time to time. Other times it won't start. Right now it is in "no start" mode. There is a fat blue spark on each plug when laid out on the cam cover. There is 120 psi compression across the board. The pressure in the fuel rail is 43psi. In "no start" mode the engine won't even fire on starting fluid. The plugs appear wet. I bead blasted them and washed them with carb cleaner but with no positive results. Feel free to make suggestions that appear to make no sense because this whole issue seems to make no sense. many thanks for taking the time to review this problem.
Bob

[erikv11]

Hmmmmm. Are you sure the fat blue spark is while it is in "no-spark" mode? I wonder if it is an intermittently bad cam sensor or crank sensor.

120 psi is really low compression, especially if it is a non-turbo, is that just the gauge?

[northernlights]

Hmmmmm. Are you sure the fat blue spark is while it is in "no-spark" mode? I wonder if it is an intermittently bad cam sensor or crank sensor.

120 psi is really low compression, especially if it is a non-turbo, is that just the gauge?

I think the cranking compression spec for a NA engine is 1.3-1.5 MPa (189-218 psi), so if you have 120 psi, that's very low. Everything together, i.e. low compression, winter (!), might be enough to cause trouble....but the fact that you said sometimes it runs OK is weird. And the fact that it won't start on ether is even weirder with the spark you describe, because ether wants to blow up!

Assuming your compression numbers are real, then you probably have a tired engine. Maybe if it doesn't start on the first try, the raw fuel that the engine dumps in on a cold start is enough to wash the cylinder walls clean to the point where your compression goes from low to really low.

So....if you are really ready for a Hail Mary - here goes!

Advancing the intake cam can increase the cranking compression, because it causes the intake valve to close sooner. There is a certain amount of adjustability on these engines because the cam wheel bolt holes are oblong. If you want to give it a shot, it may be worth checking to see where your intake cam timing is, and perhaps advancing it as far as the slots will allow.

If you look at the ipd web page, it shows a tool they sell to do this, with instructions. You can do this without a tool, but you won't know exactly what your starting point was, or your ending point.

http://www.ipdusa.com/products/7047/115 ... iming-tool

When looking at the intake cam, you want to rotate the cam clockwise to advance (or the cam wheel counter-clockwise). My suspicion is that the slots are not long enough to cause any problems as far as being able to go too far, so if you really have 120 psi, I'd go for all you can get. The IPD tool shows a range of +/- six degrees, so that's not nearly enough to cause trouble even at the limit of travel.

Starting with 10.5 compression and relatively short cam timing means that 4 degrees of added advance might give you 10 psi. The funny thing is that the engine might be timed with the cam retarded from the factory due to production tolerances, in which case you might be able to get quite a bit more.

[regent]

Low compression 'across the board' may be due to an obstructed airflow to the intake mfld. However dumb my suggestion may appear, I would like to inspect the entire air tract.

The 'sometimes' element still bothers me a lot: could there be a problem with corroded/damaged splines on the crankshaft sprocket causing 'floating timing' and intermittent no-start?

[momentumbob]

I have checked the spark on more than one occasion when in "no start" mode. It is always o.k. and the plugs are less than 8 months old. The compression is low. The car has north of 350,000 km. on the original engine but should run with 120 psi compression, albeit down on power. The compression readings are uniform so there is no cylinder specific anomaly such as a burned valve. In any case, when it does start, it fires up easily, idles smoothly and drives normally. I did dis-assemble the entire intake tract and inspect it. There is no obstruction. I didn't think to look at the crank sprocket, although the intermittency kind of militates against this possibility. I love northernlights "Hail Mary" idea. Now there is an individual that thinks outside the box. I probably won't go to that extent since the car owes me nothing and it could retire although I would prefer to keep it on the road. The situation is baffling to me and from an academic point of view I would love to know what is the cause. Thanks for your input, guys. I'll let you know if I can solve the puzzle.

[erikv11]

Is the car a turbo or non-turbo?

[regent]

I am very curious to learn what it is and I sincerely hope you have a warm garage to work in...

One other thing: I would double-check the timing marks - just one tooth off could become a contributing factor and possibly the tie-breaker in the 'low compression equation'. (I am assuming you've already done that since you mentioned the academic perspective, no?)

[Atis]

My first thought was intermittent cam sensor problem. You have spark, but at the wrong time. Although if it is a sensor problem it should set a code...hmmm

[momentumbob]

The car is non-turbo. I haven't checked the timing but when the car does start it runs with no apparent deficiencies. I have checked the codes with a code reader. I had a lean on bank one code if I remember correctly. I haven't checked the onboard code reader. The shop is well equipped (machine tools, hoist etc.) but it is COLD. I don't have the Volvo inside right now because my Expedition just puked out a spark plug so it is in the hospital. A common fault in the older Triton engines but you would have thought after 380,000 km. the truck would have been past this sort of issue. It's going to be a little while before I get back to the 850.

[abscate]

It might be worth putting oil in the cylinders to check for lawn mower syndrome.

Intermittent cam sensor is my best guess, though.

Isn't spec for fuel pump pressure 50 psi? Maybe jump fuel pump relay?