Mistimes Mags

[Cpt_Kirk]

Mistimed Mags?

One of our engines is running near 400 degrees on the ground with the power at idle. The mixture is leaned. We have replaced the CHT probe (only one) but have not touched the gauge itself. Our mechanic mentioned the possibility of having some mistimed mags which could drive the temperature up. But near 400? I don't believe that.

We have run the engine with the cowling off and shot it with a laser temp. gun with no cylinder showing above 275 on the ground. Could it just be a bad gauge in the cockpit?

 

[Dan Thomas]

One of our engines is running near 400 degrees on the ground with the power at idle. The mixture is leaned. We have replaced the CHT probe (only one) but have not touched the gauge itself. Our mechanic mentioned the possibility of having some mistimed mags which could drive the temperature up. But near 400? I don't believe that.

We have run the engine with the cowling off and shot it with a laser temp. gun with no cylinder showing above 275 on the ground. Could it just be a bad gauge in the cockpit?

Look for bad engine-to-airframe grounding. If the alternator output current can't flow freely it finds other paths, one of them being the CHT gauge and another the oil temp gauge, depending on the type and wiring. It only takes the tiniest bit of resistance to send a little more current flowing though the sender to the gauge and fooling it into reading higher.

Try a temporary jumper wire directly from the engine cylinder close to the CHT probe to the gauge case and see if that settles it down some. Or take a voltage measurement (use a low scale, maybe 2 or 3 volts) between the engine and firewall when the engine is running. Or turn a bunch of load on (lights and all) and see if the gauge goes up more.

Dan

 

[bnt83]

One of our engines is running near 400 degrees on the ground with the power at idle. The mixture is leaned. We have replaced the CHT probe (only one) but have not touched the gauge itself. Our mechanic mentioned the possibility of having some mistimed mags which could drive the temperature up. But near 400? I don't believe that.

We have run the engine with the cowling off and shot it with a laser temp. gun with no cylinder showing above 275 on the ground. Could it just be a bad gauge in the cockpit?

I'd be willing to bet there is a trim potentiometer on the back of the gauge that has been dinked with and now the system is out of calibration.

 

[gismo]

One of our engines is running near 400 degrees on the ground with the power at idle. The mixture is leaned. We have replaced the CHT probe (only one) but have not touched the gauge itself. Our mechanic mentioned the possibility of having some mistimed mags which could drive the temperature up. But near 400? I don't believe that.

Does sound rather unlikely.

We have run the engine with the cowling off and shot it with a laser temp. gun with no cylinder showing above 275 on the ground. Could it just be a bad gauge in the cockpit?

Yes. Or a problem with the wiring to the gauge (which often includes the engine grounding).

 

[Cpt_Kirk]

Thanks for the replies. I didn't know that a bad ground could do that to certain gauges. I'll look into it when I can. We should be back up there tomorrow.

 

[gismo]

Thanks for the replies. I didn't know that a bad ground could do that to certain gauges. I'll look into it when I can. We should be back up there tomorrow.

One clue to a bad engine ground is a wimpy starter when the battery is charged. You typically also see the CHT gauge go whacko while cranking if it's a resistance type sensor.

 

[BigBadLou]

Mis-timed mags wouldn't drive the temperature that high.
I would look deeper into the circuit. The trouble can be in the sensor, wires, connections or gauge. If you have multiple of each, try the elimination method, should be easy to figure it out.
And you already tried the thermal gun so that is good.

 

[Cpt_Kirk]

One clue to a bad engine ground is a wimpy starter when the battery is charged. You typically also see the CHT gauge go whacko while cranking if it's a resistance type sensor.

No issues with cranking power. I'll keep an eye on the gauge when getting it started. I'll probably record the instruments so I can dissect the video when I get home.

Mis-timed mags wouldn't drive the temperature that high.
I would look deeper into the circuit. The trouble can be in the sensor, wires, connections or gauge. If you have multiple of each, try the elimination method, should be easy to figure it out.
And you already tried the thermal gun so that is good.

I'm fairly certain almost everything in this is 50 years old. I wouldn't be surprised if it was any of that. These electrical gremlins are such a PITA.

 

[BigBadLou]

These electrical gremlins are such a PITA.

My favorite, how did you know?
Lightly roasted gremlins with pita bread .... add some fresh garlic and a little bit of salt, serve with a chilled bottle of white and you got yourself a culinary dinner.

(FWIW, I am an electrician so this has some meaning .... I think)

 

[Henning]

Thanks for the replies. I didn't know that a bad ground could do that to certain gauges. I'll look into it when I can. We should be back up there tomorrow.

You would be amazed at what corroded and missing ground straps can cause. Whenever I have to diagnose a strange electrical problem on a plane, boat, or car, the first thing I do is check the integrity of the ground plane, goes for radio problems as well. Grounds love to corrode.

No harm or foul in having him recheck his timing job though as retarded timing can cause issues, I doubt that high at idle though.

 

[Cpt_Kirk]

Well, we seem to have it under control. We fixed a few issues.

-Timing was dead on, no issues there.
-Grounding wire was fine. Wires in good condition.
-Fuel looked like Old Faithful coming out of the injector nozzles with the pump on High, tightened those up. I'm glad we had the cowling off. That stopped the shake.
-Adjusted idle speed from 1200 to 600.
-Cleaned plugs.
-Adjusted baffling.
-Laser temps showed 275 on the ground, gauge corresponded.

I guess it really needed a tune up. She's behaving now.

Thanks for the help.

 

[Dan Thomas]

-Grounding wire was fine. Wires in good condition.

-Adjusted idle speed from 1200 to 600.

Wires can look good and be bad. It's their connections that get corroded (oxidized) or loose or oily. Wire inside crimp-type connectors gets oxidized within the crimp itself. All of it can add resistance. The starter might even crank OK with a tiny bit of unwanted resistance, but guess where some of that starter current finds ground paths? Through the throttle and mixture and prop cables to the instrument panel. Not good once the grounds get corroded enough that the majority of the current runs though control cables and roasts them.

It was idling at 1200??! You must need new brake pads, too.

Dan

 

[Cpt_Kirk]

Wires can look good and be bad. It's their connections that get corroded (oxidized) or loose or oily. Wire inside crimp-type connectors gets oxidized within the crimp itself. All of it can add resistance. The starter might even crank OK with a tiny bit of unwanted resistance, but guess where some of that starter current finds ground paths? Through the throttle and mixture and prop cables to the instrument panel. Not good once the grounds get corroded enough that the majority of the current runs though control cables and roasts them.

It was idling at 1200??! You must need new brake pads, too.

Dan

1,200 according to the gauge (which I don't believe). I'll rely on my ears. They're accurate in the higher settings. 2,800 on takeoff just like they're supposed to be.

We've got it down to roughly 600. I don't trust the accuracy of the gauges below 1,000, but that's just the way they were designed (Cessna 337).