'91 740 No Hot Start Until Charge Applied

SkyVolvo · Post by **SkyVolvo** » 28 Jan 2011, 11:37

I've been having occasional starting problems with my '91 740 (178k). It is occasionally very slow to start in the morning (although this morning it started without any hesitation), but when the engine is already warm from driving, it won't start at all. Battery tested fine (although I realize that this may not be the case), but as soon as a little bit of extra charge is applied to the battery, it starts right away.

I replaced the starter cable last week, thinking this would solve the issue. It didn't. The battery terminals and connections have been thoroughly cleaned.

I've been looking through the hot-start issue list on other sites (like here: http://www.volvoclub.org.uk/faq/HotStar ... Guide.html), but nothing quite seems to fit. My fuel pump relay was just replaced two months ago, and I just changed the plugs and wires, and cleaned the cap and rotor a bit (they weren't terrible).

Why would it suddenly start just fine with a bit of extra charge?

Post by **billofdurham** » 28 Jan 2011, 12:14

When you're cranking the engine and it won't start, does the tach needle move a little? If not check the RPM sensor which is located on the back of your engine in the bell housing. The wire runs up the back of the engine compartment towards the drivers side. Look for the part number on the wire and check if the part number ends with 399. This rpm sensor has a heat related problem and it is a fairly common cause of no- start but able to start a little while later.

Now to the battery. How did you test it? On a static test it may give good readings but once you try to start it could take a nose dive.

Bill.

SkyVolvo · Post by **SkyVolvo** » 28 Jan 2011, 16:54

I was able try another hot start, and the tach definitely didn't move (other than what appeared to be vibration from the car trying to start). While I realize not many things are guaranteed...is this pretty certainly an engine speed sensor issue? If so, I'll go ahead and order it.

Once it's started, it has absolutely no issues running.

Post by **billofdurham** » 29 Jan 2011, 11:38

Since I posted the reply I have done some further digging and found that the Regina system tach doesn't move until the engine fires. Which system is yours, Bosch or Regina?

Bill.

SkyVolvo · Post by **SkyVolvo** » 29 Jan 2011, 11:50

Interesting. It is a Bosch. Out of curiosity, I may go start it just to watch the tach when it does start.

SkyVolvo · Post by **SkyVolvo** » 31 Jan 2011, 17:14

Hmm. So, even when it does start right up, when it is cranking there is no movement in the tach.

Today was an odd day for the problem: I had not driven the car since Friday, so it sat all of Saturday and Sunday. It was an absolutely frigid morning (maybe 5 degrees), and I kind of expected some trouble from it. It started right up with no hesitation.

I drove to work. Eight hours later, when I left, it REALLY didn't want to start - barely cranking, giving the kinds of sounds that normally accompany a dying battery. But I just kept cranking, and it started after maybe 20 seconds.

After a 15 minute drive home, the engine was definitely up to temperature. It never gives me any trouble once started; it idles just fine and drives with no hesitation. When I parked in my spot, I decided to try to start it to see what happened: normally, when it's gotten up to temperature, it will crank very strong but not turn over. This time, it started right up, no hesitation.

Now I'm really confused.

Post by **billofdurham** » 01 Feb 2011, 11:16

Have you checked the wire to the RPM sensor? The problem that has come up on this is a breakdown of the insulation causing intermittent short circuits.

One other thing I should have asked - have you checked for fault codes?

Bill.

SkyVolvo · Post by **SkyVolvo** » 03 Feb 2011, 17:52

I did my best to locate the RPM sensor. Am I correct in thinking that it is basically the only wired connector below the distributor? The wire I traced ended up in a three-pronged connector by the firewall, headed toward the (U.S.) driver's side. I wasn't able to check the part number at the time, but the wire didn't show any obvious distress.

I also ruled out the Radio Suppression Relay, trading it with the fan relay with no change in symptoms.

Should I be looking for other culprits? Is there any reason fuel pressure might be lower with a hot start? As I've said, it seems to crank plenty strong, runs with no strange symptoms whatsoever once started, and the starter cable and fuel pump relay were both replaced in the last two months.

Oh, and the only code it held was a 232 fuel trim code, which I cleared today.

SkyVolvo · Post by **SkyVolvo** » 03 Feb 2011, 20:00

Any chance it could be related to the ignition coil? I want to take it out and test the resistance, but the plastic on one of the connectors to the primary terminal on the left is pretty brittle - certain to crumble when I remove it. Are these connectors easily replaced from a place like Radioshack?

SkyVolvo · Post by **SkyVolvo** » 03 Feb 2011, 20:32

Hm. Removed and tested it, and was convinced it was the coil, but then I realized I had my DMM set at the wrong range. Dang.

You know, I'm thinking I need to go back and get the battery re-tested. Had it done the first time at AutoZone, but I'm not sure it was accurate.