Diesel 101

family xj

Well-Known Member
Joined
Feb 19, 2006
Location
Jacksonville NC
I need some education. I know glow plugs are a major factor when starting my 7.3. How ever after started does the glow part go away and the then the plug act as a normal spark plug? Next one is How do you get to the plugs on the 7.3? I can't even see them to get them changed. My reason for asking is that I got a code 2 months ago saying my cam sensor and glow plugs were bad. I replaced cam sensor but it still runs rough when engine is not at normal operating temp. So I am going to change the plugs now. I just looked and couldn't even see the plugs, thats why I ask. Thanks

Doug
Jacksonville NC
 
if it is a newer 7.3 glow plugs are under the valvecover, and since you suggested that the glow plug may turn into a spark plug. i don't think you are ready to go into the valve covers.
 
Glow plugs are heating elements, first and formost, they glow red hot, (hence the term "glow plug") they are only on during the first few seconds to minutes of engine run time dependant on air temp, fuel temp, and engine temp (more modern engines use all three sensors). THe glow plug sticks into the combustion chamber and the fuel injector is aimed toward it so when the injector fires, the mist ( VERY fine mist if injectors are working properly) ignites from the heat of the glow plug. once this happens, and the engine is developing it's own "heat" the glow plugs shut off.

Diesel engines run on compression ( between 15:1- 20:1 compression ratio depending on whether it is NA or Turbo)

When the piston make a compression stroke, the air is compressed so much that it "super heats" at which time the injector fires the fuel mist and ignition occurs. This happens best when temps are above 50 degrees, which is why they have glow plugs, there are some engines that have "grid heaters" in the intake that heat incoming air causing the same effect, and some engines have both.

The "knock" or rattle you hear when a diesel engine starts/runs is the ignition of each cyl. happening, it is very violent and powerful, hence part of the reason diesel engines are built so stout.

THink of this combustion "event" as detonation or "spark knock" like you would hear in a mis-timed gas engine. same thing, only in a diesel engine, you WANT this to happen.

I'll let those more familiar with the Ford diesels speak on locations
 
I am pretty confident in my mechanical abilities. I have the motor out of my Mustang GT and am getting it rebuilt. I pulled it and am putting it back in so I am pretty mechanically inclined. I am just a little unsure on the Diesel as it is new to me. It's an '01 in my Excursion and is turbo'd. Ok so once it is started it no longer uses the glow plug for heat unless it is still below 50*. If I read that correct.
So then once it is running there is no longer an ignitor (spark plug of sort) to keep it firing? It gets spark from the extreme combustion?
Did I read that correct?
I don't want to replace them unless I know I need to for sure. I hate to waste the money on something when I can use it for my Jeep. I just know that it had a code to replace them and once I fixed the cam sensor we cleared the codes and it hasn't came back. So that is why I am unsure if the glow plugs really are bad. Maybe I should replace them for preventive maintenance. It really only does it when it is cold. Suggestions any one else? Just trying to figure out what is wrong. I know it is hard to diagnose what is wrong over the web. Thanks again
 
'A' code could be one, could be several that the ECM is picking up depending on current draw. the only way to know for sure is to test each individually.

once you find out where they are, unplug each one, use a test light connect the clip to the POSITIVE post of the battery, take the probe and touch it to the spade on the glowplug, if the test light lights up, you have a working plug, if it does not light, your plug is dead, more than 3 plugs bad, replace ALL of them. or you'll be back doing them again soon.

DO NOT apply voltage to the wiring harness connecting the glow plugs, you may do damage to other electricals elsehwere in the vehicle due to a back feed.
 
family xj said:
So then once it is running there is no longer an ignitor (spark plug of sort) to keep it firing? It gets spark from the extreme combustion?
Did I read that correct?
Click to expand...

combustion is the product...... spark+fuel+air=combustion in a gas motor

diesel motor....... air+fuel+pressure=combustion....so much pressure it creates enough heat to combust i.e. explode

glow plugs help the proccess get started, they heat the combustion chamber or pre-combustion chamber so when the fuel sprays in it is already warm in there, and ignite the fuel for a brief second.

if you are having hard cold starts and it is smoking alot at startup you probably need glow plugs only weight to test is to touch them so once there might as well replace them.
 
Will it start at all on a cold morning? If not, try this, plug in your block heater. If it starts fine (whick it should) you know your plugs are not doing their job. If $ or time is tight, just plug in the truck each night. Not realy the PIA it sounds like.

Do this until you can get someone to work on it. Noone is trying to belittle your mechanical skills, but this is probably something you would want a pro involved with.

*Develop some system to remind yourself to unhook the drop cord since you are not used to it. I recomend draping the cord over the drivers side mirror
 
BlueRidgeRunner said:
Another possible problem could be a glow plug relay failing. If that relay is bad none of the glow plugs will operate. It is a common problem on 7.3s also. What year is your truck?
Click to expand...

:stupid:

This is probably your culprit and also much easier, cheaper to swap. Mine was bad when I bought my truck with only 70k on the clock. I still have the new glow plugs they just live in the glove compartment now.
 
greg slade said:
I still have the new glow plugs they just live in the glove compartment now.
Click to expand...

Do they keep the owner's manual warm? Or are they there as emergency hand warmers when waiting for the tow truck on a cold day? :fuck-you:
 
I wanted to clarify a little more. Mine starts up pretty easy on cold mornings. I have never had a hard time getting it started. I only have a problem with it sputtering when it is cold. Once it is heated up it runs fine. The sputter is very intermittent. When I say cold I mean the motor was just started and below normal operating temp. I noticed the other day when it was real cold and idling and the idle was a little high it seamed to be running better and the sputter was real minor and barely noticeable. Once I hit the throttle it idled down and did it's sputter thing.

It's an '01 with 130k or so. I am not sure if it has a block heater. Where is the plug normally located?
 
family xj said:
I wanted to clarify a little more. Mine starts up pretty easy on cold mornings. I have never had a hard time getting it started. I only have a problem with it sputtering when it is cold. Once it is heated up it runs fine. The sputter is very intermittent. When I say cold I mean the motor was just started and below normal operating temp. I noticed the other day when it was real cold and idling and the idle was a little high it seamed to be running better and the sputter was real minor and barely noticeable. Once I hit the throttle it idled down and did it's sputter thing.
It's an '01 with 130k or so. I am not sure if it has a block heater. Where is the plug normally located?
Click to expand...
The PCM will automaticall idle the engine up when the weather is extremely cold. I notice it on mine when its below 50 degrees. A rough idle is pretty normal for a diesel at first startup in freezing temps. Plugging it in may help a lot if your truck is equipped with a cold weather package. If there is a plug on it it will be located on the left front side of the frame behind the bumper. If there is a plug then you have a cold weather package. If not then your truck may not be as easy to start in cold weather. There is also a valve added to the turbo that raises the EGTs in frigid weather also. If you have it and its activated the truck will sound like a vacume cleaner when its in operation. PM me you VIN and I will check and see if your truck is equipped with that package.
 
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