Glow Plug System Seems to Work but Not Get Hot Enough to Start Engine

[rempfer]

This may have been covered elsewhere, but I couldn't find a discussion exactly like this. I apologize if this is a duplicate of another thread.

When I purchased my 1988 F250 4x4 stick shift, except for the wait to start light not staying on very long, the glow plug control circuit seemed to behave okay. The truck didn't start great but didn't appear to have any fuel delivery issue. I tested the existing glow plugs with a multimeter and discovered a couple of bad ones, so I replaced all eight with Motorcraft ZD-9's which I bench tested before installing them. Today, before making this post, I re-tested the glow plugs in the engine and they all are equal at resistance 0.1 and the harness/connectors are clean and in good shape.

Before: When I got the truck with those two bad glow plugs the dash light went out fairly quickly and the relay began to cycle but with several tries one could get the truck started. I think this behavior would be typical under the circumstances.

Now: With all (re-tested) good glow plugs I have the curious behavior that the glow plug light & supplied power seems to turn off before the glow plugs are fully heated. It takes at least three attempts to get the truck started. Interestingly, once the glow plug light turns off one never hears the relay cycle no matter how long one waits to crank the engine.

I think the difficult starting is NOT a fuel supply issue because
(a) once the truck starts it will idle smoothly with no tendency to stutter or die as it might if there were air in the fuel supply,
(b) I have a clear fuel line between the fuel filter housing and the injector return line circuit and no air bubbles appear while the engine is running or when the engine is turned off,
(c) I can repeatedly -- without cranking over the engine -- cycle the key on/off to sort of "ramp up" the glow plug heat and the truck will start more quickly when I eventually do crank it over.

So is it possible that if the glow plug circuit weren't getting enough power that the glow plug power would cycle off before the glow plugs had heated up fully and that the power cycle relay would never engage? If so, where should I look first? If not, what else could be going on?

Thank you,
****

 

[Thewespaul]

To me it sounds like your controller or relay is suspect, especially with having no clicking. How does the connector on the passenger side of the engine look? Those are know to melt due to the connector not being designed to handle the amperage the gp circuit needs.

 

[HS108]

Check to make sure you are getting proper power to the Controller and relay. Back in the day ford used 2 smaller gauge wires to feed power, check the condition, and maybe replace with one larger gauge wire

Also how fast are you turning the motor over? No issues in that aspect as well?

 

[rempfer]

The motor turns over quickly (new full size batteries), however when I tried to unplug the glow plug harness from its connector on the passenger side upper fender it resisted coming unplugged. I have a antique tractor pull to go to this weekend and so, since the truck does get SOME glow plug heat and starts even though slowly, I didn't want to risk delving into this just at this moment. If it's corroded or melted, could I just clip the ends off and permanently solder the leads together? (There must be a thread on replacing that GP connector here on site?)

Thank you, Thewespal & HS108.

-- **** Rempfer

 

[Black dawg]

How long are the glow plugs on?

 

[rempfer]

About 10 or 12 seconds when engine is cold.

 

[Black dawg]

that should be long enough, if the plugs are actually being powered that full 10-12 seconds (wait to start light and actual on time can be different), and voltage is high enough at the plug.

 

[rempfer]

Then voltage must not be "high enough at the plug." I'll have to investigate further.

The circuit breaker never coming on would probably have suggested that anyway.

Thanks, Black dawg.

 

[IDIBRONCO]

The only thing I don't like about soldering or permanently connecting those wires is that it's a PITA if you ever have to pull the engine. You now have to "unthread" the wiring harness from the engine. It's not really hard to do, but it's still a PITA compared to unplugging the plug. I have been lucky with my two 85 F250s. This plug was in good shape on both of them. When I was putting the engine back in this one a few months back, just for a safety measure, I sprayed both sides of the plug good with brake cleaner and then blew them off with compressed air. I then applied a thin coat of dielectric grease to keep build up from moisture away.

 

[BrassBandit]

Hope this helps diagnose your relay. I just had the relay on my 02 powerstroke fail this morning. Quick confirmation with my multimeter to rule out wiring or a pcm fault and 85 bucks and 10 minutes of wrenching and my glow plug system was fully operational.

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[BrassBandit]

Oh I wanted to add even if you can hear the relay click doesn't mean it is latching properly and allowing full current. The only way to know got sure is to test with a multimeter.

 

[rempfer]

that should be long enough, if the plugs are actually being powered that full 10-12 seconds (wait to start light and actual on time can be different), and voltage is high enough at the plug.

Black dawg nailed it. The "wait to start" light goes out before the glow plugs are fully heated (after about 10 seconds). If I wait another 10 seconds after the light's gone out, the relay starts clicking and the engine starts immediately. Thank you to everyone who helped -- you're the best! -- ****