Glow plug readings = strange

[rg2098]

Tell me if this sounds strange. I replaced all 5 glow plugs last year and reamed out all the holes. Now that its cold I have a really poor cold idle, as in it won't idle for about 30 seconds unless I keep my foot on the pedal.

All 5 plugs are reading a consistant 2 to 2.1 ohms. They should all be under 1. Seems strange that they would all read the same and off by that amount.

Does this sounds right?

The should be about 0.6, when cold. Was the engine completely cold? If they are all the same, I would suspect the meter is a little off.

[barry123400]

If your meter is still accurate the answer is the cold resistance appears to be at least twice as much as when new. You could check a new plug with your meter to verify it is not your meter. If you were working from one ground you might have picked up a little more resistance in the measuring loop for example. Next time. I would read one plug perhaps even with both leads on the plug. What you really need here is a measure of how much total current is being drawn through the circuit compared to a new set of plugs. Most of us do not own a meter that will handel that much current. This is somewhat academic but it could mean your plugs are only drawing about 1/2 current. A plug at 1 ohm at 12 volts is drawing 12 amps and getting pretty hot. A plug at 2 ohms at 12 volts is drawing about 6 amps. There is a feature on your meter to measure up to 10 amps or at least on most of them. If you inserted it in the line to one glow plug only it would probably verify your meter was still on calibration by indicating an initial draw of about 6 amps dropping off as the plug heats. If so the plugs are weak. Usually when heat is applied the resistance of a given element increases. Possibly the best quick test is to remove one and put it across the batttery to see how hot it is getting. Watch you do not burn yourself. If what you have posted is right the plugs would be very sub standard in heating . What brand are they? Something used should in most cases be directly compared to a new example if there is any question. Although you knew a cold new plugs typical resistance. One years functional lifespan is quite short for a set of plugs. But thats what you may find to be true with a little more testing unfortunatly. This might be a good post for you to post a little more as we do not usually get a lot of threads about the aging syptoms from this perspective on a set of plugs. Yes this problem if you verify your readings could easily give you the ratty starts but you already suspect that. Plus the subject is timely for a lot of people as the cooler weather is on it's way for some of us. It is really important as craig states to have the engine cooled to ambient temperatures for the resistance measurements to be accurate as well.

[gregszustak]

I don't know anything about glow plugs.
I know a little about electricity and heating elements.
Sounds like you're not getting any voltage to the plugs for them to get even remotely warm.

[barry123400]

Of course the last poster pointed up an important observation. It has to be assumed that there is some voltage there as most examples will not even attempt to fire with no functional glow plugs. But as always there are exceptions. Basically though if voltage is there and cold resistance is truly 2 ohms then there is a problem still that has to be dealt with unless it is just a measuring error. My thrust is I hope someone will always bring up something like the last poster mentioned. The obvious is often overlooked in examining the problem. Good call.

[carnut]

Any excess smoke when started cold? Are you manually raising the cold start idle speed with the dash control knob prior to start up? My experience with glow plugs is if not warm or some are inoperative, the car will crank too long, then start and run rough until the cylinder with the faulty glow plug warms up on its own.

[crossbones]

Hello. I have found when checking current to the glow plugs using voltage not to be the better way of do it......I find checking the amperage draw at the battery to be better.............
I hope this helps............

crossbones

Originally Posted by crossbones

Hello. I have found when checking current to the glow plugs using voltage not to be the better way of do it......I find checking the amperage draw at the battery to be better.............
I hope this helps............

crossbones

Hoe does that help you determine the condition of the individual GPs?

[rg2098]

The engine may have been a tad warm last night, I checked it this morning stone cold. They were all reading 1.2 - 1.3. I do not suspect my multimeter since it is only a year old and one of high quality (Bluepoint aka Snapon). It seems like one cylinder also doesn't want to fire right away. I just can't believe these plugs only lasted 1 year. They are Bosch BTW.

That still doesn't sound right, it's pretty unlikely they are all bad with that reading. Are you measuring with the cable unplugged from the relay? Does your meter read 0 ohms when you touch the leads together? Do you get the same reading directly on the GPs as through the cable?