Intermittent starting problem (warmish engine)

[Jase]

Hi,

Have been quiet on here, this is my first ever post on a forum so bear with me :o

I have owned my 1991 940TD since August and apart from a new battery has got through a very wet winter with no problems.

But in the last couple of weeks has been playing up a bit:

Apart from all the smoke and clattering she starts from cold okay but after driving and reaching running temp when left for a little bit (30 - 60 mins) she doesn't always start, just turns over. Often the glow plug relay sounds like it is clicking back and forth.

If anybody has any advice or information I would be very grateful.

Thanks in advance

Jason

[745 TurboGreasel]

If cold start is not smooth, you cold be looking at an ailing seal introducing air, or one glow plug failed...check fuel return line for bubbles.
I've also seen a few older injection pumps do that, and falsely triggering the glow plugs for a few seconds seem to work around the issue. As a quick check, when you are having the issue, use a short U of #10 or bigger wire to jump the 2 big terminals on the glow relay, count to 10, and have a go at starting the car.
Also if you have some type of primer pump, prime and try to start...if fixed air leak in fuel supply.

[Jase]

Hi, thanks for the info, i will try that next time!

[Jase]

Happened again today :-S Definitely seems to be a problem occurring when the engine is warm; wouldn't start after a 30min journey. We just left it, went for a walk, had a cup of tea and cake, came back and she started :-) Mind you i think it was about 3 hours!!

[ngoma]

So-- back to square one? Starts reliably when cold, starts OK when hot but not warm (after sitting 30-60 minutes)?

Low compression unfortunately can cause this. But you can "workaround" it for quite a while by forcing the GPs to energize when the engine is warm to help a low-compression start.

Simply disconnect the temp sensor for the GPs. It's the sensor at the rear of the head, almost underneath the rear camshaft cogged sprocket. This will fool the GP brain into thinking it is massively cold each time you turn the ignition key from off to on, and give max time for the GPs. You will soon get the hang of how much time to let the GPs sizzle before a warm start.

NOTE: This will tend to wear out the GPs sooner than normal.

[Jase]

Hi Ngoma, thanks for the info. I díd vaguely remember something about the temp sensor, but in the middle of the car park with my little son getting fed up in the back couldn't see it! :$ i'm putting in new gp's today well hopefully....

[ngoma]

That must mean you have verified the GPs are bad?

[Jase]

No, I bought the plugs on some earlier advice. I thought i may as well fit 'em; as it turns out no 6 plug hadn't been done with the last change.
Car wouldn't start again last night, disconnected the temp sensor and off she went :-)
Thanks again

[ngoma]

Great! You can test the old GPs and keep the good ones as spares. Use a large wire size as they pull high amperage. I like to use battery cables, powering them from a car battery. They should glow red hot within about 6 seconds or less.

Too bad about the apparent low compression, which is what this symptom exhibits (cold/hot starts OK, warm starts require GPs). When was the last valve clearance check/adjust? Driving around with the valves too tight can burn and erode them.

[Jase]

Car will not start this evening even with glow plugs :-( Not sure about the valves, i'll have a look at what history i have.

[ngoma]

Car will not start this evening even with glow plugs :-(
Was that a warm start attempt?

[Jase]

Yes it was a warm start.

[745 TurboGreasel]

Check for air bubbles in the fuel return before you go too much further .

[Jase]

Will do, once i find it! :-D
Thankyou

[Jase]

No bubbles in clear pipe, hoses seem okay and secure. :confused:

[ngoma]

Where is that clear section? Can't tell but looks like it is in a low section of the return hose? If so, there may be bubbles exiting the IP but you won't see them at a lower hose section because they "pool" and coalesce in a higher (highest?) portion of the return fuel line.

We use evidence of air in the IP return to infer air intrusion into the IP, where there should be no air. Air in the IP most often points to two failure modes:

1) Failing IP mainshaft seal allows air intrusion while engine is not running, which subsequently causes hard start condition. Not usually a problem once running, as that section sees (minor) fuel pressure when running, not vacuum. It may leak fuel when running.

2) Loose connection or leaky hose in fuel supply run to IP. Or the presence of an extra small fuel filter sealing o-ring. Often the smaller of the two sealing filter o-rings is not seen and therefore not removed during a filter replacement. Since a new o-ring is usually supplied on the new filter, the filter gets installed with two o-rings sandwiched together, causing air intrusion into the IP. This failure mode can cause hard starting, and almost certainly poor performance, especially under WOT (high fuel demand) conditions.

[Jase]

I put a new air filter on and double checked the o-rings, there has never been a problem with performance either

The injection timing has been reset as someone had tried to advance the timing.

Should the lever that is adjusted with the cold start device be free moving? Should you be able to move it by hand? Seems to be stuck tight.

[745 TurboGreasel]

they are very stiff with the engine off.