850 won't start after short drive & short rest

mc510 · Post by **mc510** » 07 Oct 2011, 16:35

I've had an intermittent problem with my 1996 850 wagon (base model, no turbo) sometimes not starting. I think I've finally figured out the set of conditions: Cold start at home, then a short (5 to 10) minute drive on local streets, then short rest (often in the parking lot of Home Depot) for 10 or 20 minutes. Then I try to start it and it just cranks with almost no indication that it's trying to catch. After another 10 or 15 minutes of resting it will start up with just a little more cranking than usual, and then seem to run fine. These conditions don't guarantee that it won't start, but it seems that every time it happens, these conditions are met.

No codes. There's fuel pressure at the schrader valve on the fuel rail (tested when having the not-starting problem). MAF has been cleaned. Got about 180,000 miles.

Does this sound familiar to anyone?

FCPEURO · Post by **FCPEURO** » 07 Oct 2011, 17:01

Sound like a bad fuel pump relay. Have you tried jumping the relay when this problem comes up? Can you hear the fuel pump running when you put the car in the on position?

mc510 · Post by **mc510** » 07 Oct 2011, 18:27

I haven't tried anything with the fuel pump relay; I've been assuming that fuel isn't the problem, because I find pressure at the fuel rail even when it won't start. I'm not entirely sure what the fuel pump sounds like, but when I put the key in the on position I do hear a low whirring for about 2 or 3 seconds. I suppose I'd guess that I'm hearing the fuel pump, but it's only a guess. Anyways, this sound is present even when the car won't start.

Post by **jreed** » 08 Oct 2011, 07:08

If you haven't pulled the spark plugs already, it might be helpful to check the electrode shape and gap to make sure they are OK.

mc510 · Post by **mc510** » 09 Oct 2011, 22:59

jreed, I haven't thought of that, but I'll check it out.

I've been poring through old forum discussions; seems like most of the things that cause warm-start problems either will set a code or are in the fuel system. With me getting no codes, and (I think) having normal fuel pressure, I'm kind of at a loss. I hope the spark plugs are the thing!

mc510 · Post by **mc510** » 11 Oct 2011, 13:44

My most accomplished shade tree mechanic pal thinks it may be related to bad sensor in a cold-start circuit:

I would guess that there is a cold start mode of some kind that gives the extra gas or an extra hot spark when it’s cold. When you first start it, this works well. But, when the car is partially warmed up, the temp sensor is not working right (just guessing here) and so it is either giving it too much gas so it floods or not enough (thinking the car is already hot) so it won’t start. My Alfa had this problem, and it was fixed by replacing the temp sensor for the fuel injection cold start circuit.

Is there something similar in Volvo 850 terms? There's a coolant temperature sensor, right? But I gather that it's supposed to throw a code when it fails.

mc510 · Post by **mc510** » 16 Oct 2011, 14:44

jreed wrote:If you haven't pulled the spark plugs already, it might be helpful to check the electrode shape and gap to make sure they are OK.

Just finished installing new plugs. The old ones were really worn with an oversize gap. Not even sure how long they've been in there. Don't know if this is going to fix my warm-start problem, but those plugs were overdue for a change regardless!

gilhuly · Post by **gilhuly** » 16 Oct 2011, 20:54

I believe your buddy is correct and you may have a bad coolant temp sensor. One way to be sure is the next time you have the problem pull the fuel pump fuse under the hood (#2 I believe) and the car will start after working off the excess fuel and run for 5 seconds or so til it burns up the available fuel and stalls.

The CTS is on the thermostat housing and has an electrical lead. Long torx bit is best for the thermostat housing. Do the thermostat while your there.

Post by **vbel** » 01 May 2012, 18:50

Hey,
Were you able to figure out where the problem was coming from. I have the exact same thing in my c70 and I am trying to solve it. Thanks

mc510 · Post by **mc510** » 01 May 2012, 22:48

Hey vbel, it kind of seemed like new spark plugs actually did fix the problem. At least it didn't reoccur at all for maybe 5 months (right up to the point where it badly overheated on the freeway and cracked the head, leading to a one-way trip to the junkyard). So if you haven't pulled the plugs yet and checked the gap, it's worth a shot!