Cold weather starting... Glow System checks

[Lonnie]

I know, I know, this has been covered before, but I've read several threads on this subject but nothing really fits my problem.

First thing in the morning the wait to start light stays on for about 40 seconds & as soon as I hear the thunk I turn it over. It takes about 8 - 10 seconds of cranking before the engine will stay running. Even at that it runs like one or two of the cylinders aren't fireing & it smokes like crazy. It takes about 45 seconds before it will somewhat smooth out & after it does it runs great!

I figured one of the glow plugs were bad but I pulled them all, tested them & they glow bright orange after about 10 - 15 seconds. I believe the afterglow system is working because this morning after it fired up it didn't stay running & as soon as it stopped the light came back on without turning the key off & back on...

Anybody have any ideas?

Thanks for your help!

Lonnie

[asavage]

Philip likes to say "leaking down injectors". EvergreenSD and zen like to say "retarded timing".

Even though you have GPs that glow, I am still suspicious of the GPs and the voltage to them. Take a volt reading at the terminal at the No. 1 GP, cold engine. Turn the key to ON and read the voltmeter. Is it less than 11v? If yes, is the voltage at the battery's pos post above 11v? If not, you don't have enough battery.

The spade terminal at the GP bus is a bad one -- I can't seem to find my picture of it, but follow the GP bus wire to the back of the head and you'll find it. Some people have complained about the GP relay too.

One of the trouble spots is here:


Another is the fusible link -- I've had two SD22 fusible links fail for the GP system.



As I said, check voltage at the end of the GP bus first (No. 1 GP), and make sure you're getting close to battery voltage there.

[philip]

SNIP- I believe the afterglow system is working because this morning after it fired up it didn't stay running & as soon as it stopped, the light came back on without turning the key off & back on...

Lonnie

The oil light would come ON again. For the glow plug light to come ON again would require turning the ignition key OFF then back ON.

Do read up on the DPC and Fuel Control Lever bushings.

Do confirm correct injection timing using the method outlined in your FSM.

Your glow plugs seem to pass muster. The glows normally come ON again after the engine starts ... with the duty solenoid shutting OFF (thunk) about 30 seconds later. If you have misfiring during this 30 second period, there may be air getting purged from the injector lines or you are raising the RPM too much too soon.

You cannot "clear out" one of these diesels by racing it right after cold starting. Wake the little fella gently.

[moose60]

In addition to everything already mentioned, you may be able to hear the GP relay "thunk" off a second time when the after glow period is finished (~30 sec after startup).
What temps are you experiencing this at? Is there any associated surging, or fluctuations in engine speed with a constant throttle position?
And the tough question: Do you have good compression?

On the subject of air in injector lines. People have mentioned to me that the priming pump may admit air to the injection pump while the vehicle is off, causing some of the rough running Philip mentioned. Some Mercedes folks will tell you that a primer pump that leaks in use must be replaced. I have also heard people say if it does not seem to leak when locked (screwed in) just leave it. They are not very expensive, and are available in a newer "leak proof" style.


Philip is also correct about keeping the revs low at first. Makes a big difference on mine.

[Lonnie]

At this point I can have the throttle half way down & it's running so bad the RPM's won't pick up until 10 or so seconds have passed.

The temps have been between 20 - 40.

It doen't take very long before it clears up & starts running smooth & that's what prompted me to check the glow plugs. It really seems to act to me like a GP issue...

I'll check the voltage tonight.

Thanks,
Lonnie

[Lonnie]

Tonite I did a quick test like you outlined Al.

The battery reads 12.58 volts. GP #1 10.20 volts, ding, ding, ding!
I didn't have time to go any futher but I'll run down why I've not getting enough voltage...

Tomorrow is a big day for the old girl. All new ball joints, idler arm, brakes & rotors. She won't know how to act!

I went to put a new set of tires on her last week but someone had lowered her 3"! So I had to take the spacer off of the back & tighten the front torsion bars. The tires were 3"shorter then stock so I put a new set of mud & snow stock size tires on. After I get through with the front end I'll get an alignment.

Thanks to everyone for the help!

Lonnie

[Lonnie]

Me again,
Ok, so I checked the voltage at the hot side of the GP relay & I get the same voltage as the battery. 12.58v

Then I turned the key on & checked the voltage on the other side. 10.5v.
So I unhooked the GP side of the relay & tested that post with out the wire hooked up. 12.4v.

I cleaned the terminal & hooked it up again & I'm back to 10.5v. Number one GP reads 9.9v to 10.2v.

Now I'm confused! Will there be this much voltage drop with the system hooked up or is there a reason for that much difference?

Ideas anyone?

Thanks,
Lonnie

[asavage]

You have to make your tests of the two terminals of the GP relay under the same test conditions: loaded. That is, you need to make the test of the voltage on both sides of the GP relay while the GPs are pulling current.

Even a crap'd relay will measure battery voltage on both sides when energized if there is no load on it.

My suspicion is that your battery is bad, or one of the battery terminals is dropping the voltage (esp. if you have those temporary replacement "repair" terminals for top-post batteries). Try measuring four places all in one test cycle, with the GP relay energized (whether by the Ign switch -> GP controller, or a hotwire to the GP relay's green lead if you prefer):

* Battery post
* Battery terminal
* GP Relay (battery terminal)
* GP Relay (GP bus terminal)

The battery post should read above 10.5v minimum (actually, I like to see 10.5v min. while cranking). If it doesn't, you probably need a better battery before you can do much. But you can take the other readings too. They should all be within a couple of tenths of a volt of each other.

[philip]

Me again,
Ok, so I checked the voltage at the hot side of the GP relay & I get the same voltage as the battery. 12.58v

Ok

Then I turned the key ON & checked the voltage on the other side. 10.5v.

As it should be with the glow plug wire still connected until ... the glow plug controller cuts juice to the relay (30 secs).

So I unhooked the GP side of the relay & tested that post with out the wire hooked up. 12.4v.

Until the glow plug controller times out, then zero volts. You do NOT have an electrical load present with the glows disconnected. All you confirmed was continuity.

I cleaned the terminal & hooked it up again & I'm back to 10.5v. Number one GP reads 9.9v to 10.2v.

Now I'm confused! Will there be this much voltage drop with the system hooked up or is there a reason for that much difference?

Lonnie

Well duh. You "hooked it up again" which means you restored the electrical load of the glow plugs. This circuit is ok. When load (amperage) increases, voltage decreases.

Maintain the engine at IDLE right after it starts... for at least a minute. By racing a stone cold engine, you are douching the glow plugs with cold fuel ... there goes needed ignition heat!. You are after four cylinders firing at 700 rpm ... not 2 cylinders firing at 2,000.

[Lonnie]

Thanks Al,
It kind of makes sense now. I had a local shop run a load test on my battery shortly after I bought the pickup & they told me it was very marginal.
I tested each place you said & all but the GP itself showed 10.2v. At the GP it tested 9.9v.
So it looks like a new battery is needed...
Oh well, nothing like getting everything done at once!

Lonnie

[asavage]

0.3v drop across the OEM GP bus is probably OK. IMO, it's not a very heavy bus, and with two spade connectors in series between the GP Relay and the GP bus, it's amazing that it works as well as it does.

Spade terminals are just not up to the kind of load put on them in that circuit, over the long term.

Buy the largest CCA you can afford and that can fit in there -- remember, several diesels (even Fours) have two batteries paralleled to get enough CCA to start them.

And for Gawd's sake, stay away from Exide batteries. I like and buy only Interstate, and the Mega-Tron Plus is their top-of-the-line (until you get to the AGM batteries (like "Optima"), which are a different line altogether).

Using Interstate's online app. guide, I see both the MTP-27 (750 CCA) and MTP-78DT (800 CCA). I have a couple of the MTP-27s in my two Maxima Wagons, they're good but they're also long. The MTP-78DT is 1.5" shorter, which will fit much better in the 720.

This afternoon, I stole the MTP-27 out of the '83 Wagon to start the '82 720 ("Bellingham Truck") to move it off the tow dolly and into the shop, prepatory to dismantling it. The MTP-27 will fit, but it doesn't leave a lot of room above the IP.

There is no wiring harness in the truck, I sold it to Andy in AZ last month. But the battery cables (ground and to starter) are still there, as is the basic GP bus. The truck and the battery have both been sitting outside and are at about freezing. The truck hasn't been started since about April.

I bolted on the terminals, put a light clip lead on the GP bus and connected it to the battery. The clip lead is really too light for this duty, but I let it stay connected while I fiddled around connecting a longer clip lead to the starter. Got that hooked up, thought I'd better make sure the starter still works, so I tapped it to the battery. Yup, the starter works, and the damned thing started. I hadn't had the GPs running maybe 20 seconds. I was surprised.

Anyway, Interstate batteries aren't cheap, but they are very good. Search this site for "Interstate" for more info.

MTP-78DT - Mega-Tron Plus - 85 Months - 800 CCA

"DT" = Dual terminal, ie both top and side terminals.

[asavage]

0.3v drop across the OEM GP bus is probably OK. IMO, it's not a very heavy bus, and with two spade connectors in series between the GP Relay and the GP bus, it's amazing that it works as well as it does.

Spade terminals are just not up to the kind of load put on them in that circuit, over the long term.

They are connectors 37M & 35M
(1982 FSM, page EL-41)



Those two connectors, and the one at fusible link 33M are the trouble spots.

[Lonnie]

Thanks Al, you're a great help!
Elecrical isn't one of my strong points as you can tell!
I'm going to get a new battery being this one I have is on the edge anyway & I'll check these two points.

Right now this is all kind of on hold being I'm in the middle of rebuilding the front end...

Lonnie

[pbknowles]

Just a couple more thoughts on cold starting from my experience:
1) Air leaks into the fuel system can be very sneaky, and can cause rough running after initial start, like you describe. I was sure that I had eliminated air in my fuel system, but after I copied Phillip's mod to make my I.P. self bleeding, my cold running improved, indicating that there was still air hanging around in the I.P.
2) Advancing the pump can help, a lot, but be careful! My experiments indicate that if you advance the pump enough you can get the engine to start and run clean no matter how cold it is. But you definitely wouldn't want to actually run it this far advanced under load, you need to find a happy medium and err on the conservative (retarded) side if anything. These engines really need a cold advance mechanism of some kind, but I haven't figured out how to engineer one.
Here is a pretty good synopsis on why not to run a lot of pump advance:
http://www.oliverdiesel.com/tech/timing.htm

[philip]

continued ... If the safety wire on the smoke screw is broken, someone dinked with it. I adjust mine based on full load EGT, but if you don't have a pyrometer, try decreasing it until you noticeably lose power under load, then go back a bit.

Curious ... what was the actual exhaust gas temperature you settled on? Under what engine load conditions (ie, 3rd gear, full load, 38 mph, hill). Visible smoke in the rear view mirror or not?

Turning the smoke setscrew clockwise decreases fuel decreases fuel delivery ("turns down" power/smoke). It's a pretty sensitive adjustment as you found out. 1/4 turn is quite a bit. Also, if the altitude compensator is touching the Fuel Lever then you must first back it off before attempting to "turn down" (lean) the fuel lever position.