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Old 08-22-2016, 02:47 PM   #1
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Driving with failed glow plugs?

Hello Everyone,

I am considering the purchase of a rig with a 7.3 diesel. The glow plugs are failed. I have no idea if it is the glow plugs, harness or relay at this point.

If I buy it I will have a 800 mile drive home from the Bay area to Seattle. Interstate driving and warm weather.

What are the potential problems driving a 7.3 equipped rig with failed glow plugs?

My Cummins 5.9 does not run the grid heaters (instead of glow plugs) in warm weather and has no issues but I know very little about the 7.3's.

Am I safe to drive it in this condition?

Thanks

S.
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Old 08-22-2016, 04:27 PM   #2
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The primary thing the glow plugs do is make it easier to start, especially when cool, and in cooler weather. Once running and warmed up, the glow plugs don't do much of anything.
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Old 08-22-2016, 04:44 PM   #3
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Originally Posted by Brad_SwiftFur View Post
The primary thing the glow plugs do is make it easier to start, especially when cool, and in cooler weather. Once running and warmed up, the glow plugs don't do much of anything.
That is my understanding.

My Cummins has grid heaters instead of glow plugs but they perform a similar function. In warm weather they don't run or only run for a very short period. I know that if they failed I probably would not notice until the weather cools somewhat.

What I don't know is how the Ford would behave in similar circumstances and if there are any other potential ill effects.

Thanks for the reply.

S.
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Old 08-22-2016, 05:04 PM   #4
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Glow plug/grid heaters are only for starting. Actually glow plugs are off once the glow timer times off, before you engage starter.
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Old 08-22-2016, 05:32 PM   #5
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Glow plug/grid heaters are only for starting. Actually glow plugs are off once the glow timer times off, before you engage starter.
That is my understanding.

My Cummins has grid heaters instead of glow plugs but they perform a similar function. In warm weather they don't run or only run for a very short period. I know that if they failed I probably would not notice until the weather cools somewhat.

What I don't know is how the Ford would behave in similar circumstances and if there are any other potential ill effects.

Thanks for the reply.

S.
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Old 08-22-2016, 05:42 PM   #6
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You don't want glow plugs on anything to be on when the engine is running, it will cause preignition. Like pinging in a gas engine.
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Old 08-22-2016, 05:45 PM   #7
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You don't want glow plugs on anything to be on when the engine is running, it will cause preignition. Like pinging in a gas engine.
Shouldn't be a problem on this one as they don't work.

Anyone aware of any potential issues with running the rig 800 miles without them in reasonably warm weather?

I wouldn't think that there is but there are things that I don't know and I prefer not finding out that I was wrong the hard (expensive) way.

Thanks.
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Old 08-22-2016, 06:28 PM   #8
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Should be no issues running that 800 miles in warm weather. Or even cold weather once the engine is started. You'll be fine, they're off when the engine is running.
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Old 08-22-2016, 08:04 PM   #9
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Cal's shorty should be fine. Tell him I sent you and get a big discount!
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Old 08-22-2016, 08:13 PM   #10
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Quote:
Originally Posted by somewhereinusa View Post
You don't want glow plugs on anything to be on when the engine is running, it will cause preignition. Like pinging in a gas engine.
As I understand it, glow plugs on the 7.3L do stay on for 30 seconds or so after the engine has started. When watching my digital battery monitor I see my starting bank voltage staying down around 11 volts until I get out the driveway, then it pops up to 14.x volts.

As everyone here said, no problem driving with dead glows. It may be harder to start but once you're running it's all good.

If your bus has OBDII diagnostics many auto parts stores will scan your error codes for free.
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Old 08-22-2016, 08:28 PM   #11
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Good luck! Maybe a long test drive to get it up to temperature before handing over the cash?
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Old 08-22-2016, 08:28 PM   #12
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as long as the engine starts I say drive it all day!... some engines like mine dont even have glow plugs or grid heaters.. they arent required for Diesels.. its summer you probably wont even have a slow start... worst case it might smoke for a little bit till all the cyclinders warm up and get firing cleanly...

if you have a block heater you can plug it in and fully warm the engine which will ensure it should start right off.

Diesels wont "ping" or pre-ignite as the timing is controlled by the fuel charge... unlike a gas engine, theres no fuel in the cylinder during a good portion of the compression stroke.. so therefore even a stuck-on glow plug has nothing to burn...

it COULD cause ever so slight early ignition when the fiuel charge is injected just because it might accelerate the flash-over a bit but usually the only damage is the glow-plug itself gets damaged from over-heating the plug..

-Christopher
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Old 08-22-2016, 08:33 PM   #13
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Quote:
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Cal's shorty should be fine. Tell him I sent you and get a big discount!

Oh come on now!

He asked if I knew you and said that if I did he would charge me double!!
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Old 08-22-2016, 08:44 PM   #14
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Quote:
Originally Posted by cadillackid View Post
as long as the engine starts I say drive it all day!... some engines like mine dont even have glow plugs or grid heaters.. they arent required for Diesels.. its summer you probably wont even have a slow start... worst case it might smoke for a little bit till all the cyclinders warm up and get firing cleanly...

if you have a block heater you can plug it in and fully warm the engine which will ensure it should start right off.

Diesels wont "ping" or pre-ignite as the timing is controlled by the fuel charge... unlike a gas engine, theres no fuel in the cylinder during a good portion of the compression stroke.. so therefore even a stuck-on glow plug has nothing to burn...

it COULD cause ever so slight early ignition when the fiuel charge is injected just because it might accelerate the flash-over a bit but usually the only damage is the glow-plug itself gets damaged from over-heating the plug..

-Christopher

Thanks Christopher,

That is pretty much what I expected but I have been bitten in the backside more than once by details that I did no know beforehand.

I have driven WAY more miles on a diesel without any sort of glow plugs than I have rigs with. I did have to slide my BBQ under the oil pan of my 8v71 one February in South Dakota.......

I think I have a good idea now that there are no hidden surprises to be found by driving a 7.3 without working glow plugs.

Thanks Everyone for the input.

S.

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Old 08-22-2016, 09:23 PM   #15
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The first thing to check is the easiest + most prone to failure. The glow plug relay. Its just a common Ford starter relay. You can test it easy with a test light. Theres 2 large terminals-one will be on all the time with the key on-the other should be on for 2 minutes. As roach said -you can see it change on the voltmeter. Its located on the passenger side-above the front of the valve cover.

I've gotten by with a few dead injectors-it just takes a bit more cranking to start. stone cold engine and 50 degrees in might take 30 seconds of cranking.
The best way to get the most out of what plugs are left-leave the key on for 1 to 1 1/2 minutes. The glow plug timer is 2 minutes-the "wait to start" light on the dash is only on for like 15 seconds. And its just a simple timer-not hooked to the plugs at all.
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Old 08-22-2016, 09:32 PM   #16
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And if you do change glow plugs, do NOT use Autolite!. I didn't 2 years out of my last set. And the sentiment on the Ford diesel forums I go to is the same .You can get factory Ford ones on ebay for about $80 a set.
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Old 08-22-2016, 09:54 PM   #17
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My DT466 doesn't have glow plugs... But it's got a liquid ether squirter that doesn't look like it's been used in a while, but the electric pump works
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Old 08-23-2016, 04:26 PM   #18
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Glow plugs on newer engines will continue to operate for a few minutes after engine start when it's cold out. The light on the dash going out doesn't necessarily mean the plugs are off.

As far as pre-ignition goes. That's false. Ignition happens based off the injection of the fuel. It might start a hair or two earlier with the plugs still on but it won't be near as bad as pre-ignition in a gas engine. The worse that will happen is you over heat the plugs, which will necessitate replacement, which can be a pain depending on how bad they're damaged.

Depending on if it's a DI or IDI engine will determine how hard it will be to start. You might notice a little hesitation in a DI engine when starting but it should still fire up during the summer months. An IDI engine relies a lot more on the plugs for starting so it might struggle depending on compression, injector health, and outside temp.

If you're interested in diagnostics, I can help, but more info is needed.

So what engine is it?
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