850 T5 runs fine, then won't start for a week.

[SomeGuy537]

Hi, everybody. There is an 850 T5 for sale in my area that supposedly runs fine and then refuses to start for a week. The owner claims to have replaced a lot of parts on the engine only to have the issue remain. I'm considering buying it if i can get a good price but first i would like to get a clue about what might be wrong with it. My coworker who is an educated mechanic says it might be the ECU. If this is the case could anybody tell me if it's easy to replace it. Do you just remove an ECU from a scrap car, install it and drive or do you have to program it into the car or do some other fancy computer stuff? It's a 94' by the way.

[misha]

Just find exact the same ecu with exact same Bosch numbers.
Also pay attention if you have manual or auto transmission...you can't use ecu from manual to auto car and vice versa(you can,but it will not idle properly and you will have some minor driving issues).

Remove the old one....put in the new one and that's it...just make sure that key is removed from ignition.
No programming and mating with the car required.
Just plug & play.

[rguzz]

There are some that would say it's almost never the ECU, and some that would suggest avoiding a 94, 95 because of their oddities. Not exactly answering the question you asked I'll admit.

[Roger_850T]

Does it refuse to start for a week, and then just run fine again? Or did it stop running a week ago and has not started since? (i.e. is it an intermittent problem, or did it just fail a week ago and he hasn't figured out what it is yet?)

Before throwing an ECU at it, (like the other guy just threw some random parts at it,) I highly recommend you do some testing first. Yes, I'd take this on - this is how I bought my last Volvo!

Check for fuel pressure at the rail. There's a schraeder valve, so it's easy. It should come up to 3 bar, 43.5psi, when you first turn the key on.

Check for good spark when you crank it. If it has the distributor on the head, (i.e. not coil packs,) then it's easy to clip an inductive timing light on to one of the wires and confirm that spark is happening.

Check compression. Too low across the board suggest the timing belt has slipped or is otherwise not correct. Too high across the board suggests an exhaust blockage. (Yes, I've had it where the catalytic converter broke up internally and plugged the exhaust. That's how I bought my second Volvo.)

When you check compression, note the condition of the spark plugs. Are they wet with gasoline? New-ish and in good condition? Or have the been in there for a few eons?

There's a condition referred to as the "lawnmower syndrome" with these cars. If you start it cold and run it for less than a minute, (i.e. you move the car to get the lawnmower out,) it can happen that the excess fuel from the cold start can wash down the cylinder walls and result in poor compression. (Wet plugs and low compression would suggest this.) This can be remedied by adding a bit of oil to each cylinder to help the compression a bit, then cranking - a lot. Eventually it will catch and run. You can be on the lookout for this possibility, especially if the po started it and moved it then shut it off right away, never to run again.

Check all these things, then let us know!

Also, what year? and NA or Turbo?

Good luck!

Roger

[abscate]

It is almost never the ECU, I think we literally have had one case here of an ECU replacement solving a car problem, in 20 years.

[SomeGuy537]

Does it refuse to start for a week, and then just run fine again? Or did it stop running a week ago and has not started since? (i.e. is it an intermittent problem, or did it just fail a week ago and he hasn't figured out what it is yet?)

Before throwing an ECU at it, (like the other guy just threw some random parts at it,) I highly recommend you do some testing first. Yes, I'd take this on - this is how I bought my last Volvo!

Check for fuel pressure at the rail. There's a schraeder valve, so it's easy. It should come up to 3 bar, 43.5psi, when you first turn the key on.

Check for good spark when you crank it. If it has the distributor on the head, (i.e. not coil packs,) then it's easy to clip an inductive timing light on to one of the wires and confirm that spark is happening.

Check compression. Too low across the board suggest the timing belt has slipped or is otherwise not correct. Too high across the board suggests an exhaust blockage. (Yes, I've had it where the catalytic converter broke up internally and plugged the exhaust. That's how I bought my second Volvo.)

When you check compression, note the condition of the spark plugs. Are they wet with gasoline? New-ish and in good condition? Or have the been in there for a few eons?

There's a condition referred to as the "lawnmower syndrome" with these cars. If you start it cold and run it for less than a minute, (i.e. you move the car to get the lawnmower out,) it can happen that the excess fuel from the cold start can wash down the cylinder walls and result in poor compression. (Wet plugs and low compression would suggest this.) This can be remedied by adding a bit of oil to each cylinder to help the compression a bit, then cranking - a lot. Eventually it will catch and run. You can be on the lookout for this possibility, especially if the po started it and moved it then shut it off right away, never to run again.

Check all these things, then let us know!

Also, what year? and NA or Turbo?

Good luck!

Roger

It's a T5 and it apparently starts and runs fine and then it cranks with no start for a week and then returns to normal again. The guy has replaced spark plugs, cam position sensor and checked all relays and they work. Does anybody know if a 94' turbo engine have the simple button and LED diagnostic system that my 94' NA has.

[misha]

'94 have obdI diagnostic ports under the hood just like any 850 up to '96.
All you need is a pencil and a piece of paper to write down the flash codes.

[jreed]

Agreed with all the items Roger 850T suggested above. You mentioned that a new camshaft position sensor was installed... If it hasn't already been done, it would be a good idea to check or replace the crankshaft position sensor too.

[abscate]

Diagnostic codes rarely help a no start.

Compression
Ignition
Fuel
Air
Timing