Chaotic Engine Temps

[yeahmag]

I recently changed out a head gasket due to failure and rebuilt the carb as a result. I also did an acid flush and added the MM thermostat (late engine) and now finally have temperature showing on my gauge. What's concerning, is the temperature fluctuates all over the place. It will swing from 160F to 200F or more. Sometimes settling at 180F for a bit before going all over the place again. I have a good flow of water out the back and when it was "hot" I reached over the stern and felt it. I'd say "warm" at best. Motor runs great. Plenty of power and doesn't miss a beat. The motor is raw water cooled and in Southern California. It gets a fresh water flush after every outing.

I'm thinking I have one of two issues. The sender or gauge is failing or I need a restriction in the recirculation line from the thermostat.

Has anybody experienced this before?

Thanks!

-Aaron

[JOHN COOKSON]

Welcome to the forum.
I don't have a definitive diagnosis to offer. Here are some thoughts for your consideration.
Check the squirrelly temp gauge readings with an IR thermometer. I doubt that the gauge readings are real but this would be a definitive test.
The sending unit wiring may have some sort of weird intermittent reistance in it. Does the sending unit wiring go through any sort of plug between the sending unit and the gauge? These plugs are always suspect. You could run a jumper wire sending unit -> gauge, bypassing the current wiring, and see if the jumpy readings away.
How clean is the water inlet strainer? Growth could cause some sort of intermittent blockages.
Have a look at the water pump impeller. Sometimes bits of the impeller break off and get lodged in the cooling system and cause all sort of weird symptoms.
Remove the sending unit for a look see. One time when I removed the temp sending unit there was a lot of mineral deposit on it which I got rid of with lime away and a tooth brush.
Note: Don't go after the sending unit with a open end wrench. You will round the soft brass. I had good luck with a closed end wrench and hammer. If the sending unit does not yield start the engine and get it warmed up and give it a try. The heat + vibration will help to loosen the sending unit.
Others will have more ideas to try.

ex TRUE GRIT

[Peter]

What is the time scale for these fluctuations?

I would think that any real temperature changes would be slowish - degrees per minute as opposed to degrees per second

Another simple test would be a meat thermometer taped somewhere on the hose leading to your water injection. If the fluctuations are real you should see them there as well.

I am also betting a sensor/gauge/wiring issue.

Peter

[ndutton]

If you want to diagnose this logically - - and I think you do - - you need to determine if this is an engine cooling problem or a gauge system problem. With an IR (infrared non-contact) temperature gun, readily available at Harbor Freight Tools, you can take measurements all over the engine as it warms up over a 30 minute period and compare them to a table of measurements taken during the new thermostat studies which I have attached. Not only will it indicate if there is a cooling system problem but also where the problem might be. We can help with the analysis of your table once completed. I have also attached a blank table for your use. The third attachment is a gauge system troubleshooting guide.

[joe_db]

I had an engine that would do that. It was slowly dying of salt water corrosion and I think pieces of metal were blocking various passages and then water/steam pressure would blow them out. It could go from pegging the gauge to 120 and back pretty quickly
OTOH a bad gauge, wiring, or sender seems more likely. I would check that first!

[Al Schober]

The temp sender on the head over the flywheel seems as likely to give you head temperature as water temperature. The nose of the sender may or may not get good water flow. I spent some time with an IR thermometer before deciding to move the sender to the water outlet of the manifold. I was happy with it there and didn't move it back.

[yeahmag]

Neil,

Thanks for the worksheets, example, and tips. I'll try and do this next time I'm down. I've got an IR gun and lot's of experience with motors. Unfortunately, most of that experience is with air cooled stuff (Porsche/VW).

Looks like you're in San Pedro as well. Morning Star is on D Dock at CBYC. Are you close by?

-Aaron

[ndutton]

Looks like you're in San Pedro as well. Morning Star is on D Dock at CBYC. Are you close by?

Berth 31, E dock, Cabrillo Marina (behind the Doubletree Hotel, in front of Los Angeles Yacht Club).

[Al Schober]

Funny you should mention air cooled stuff, because they are the only folks that really seem to worry about head temperatures. Porsche/VW are one group, but a group with more research are the ultralight fliers - probably because they can't just pull over to the side of the road to let things cool off. Their limits on head temperatures vary depending on the engine, but are in the range of 450-550 F.

[joe_db]

Funny you should mention air cooled stuff, because they are the only folks that really seem to worry about head temperatures. Porsche/VW are one group, but a group with more research are the ultralight fliers - probably because they can't just pull over to the side of the road to let things cool off. Their limits on head temperatures vary depending on the engine, but are in the range of 450-550 F.

What I like to fly with:

The bars are EGT, the gap in the bars is CHT, and the digits at the top are TIT.
It isn't just ultralights, any piston engine airplane benefits from CHT instruments. A 2-stroke engine lives and dies on EGT, by the time the heads heat up you already ruined it

[yeahmag]

I data log with an Innovate Motorsports LM-2 and use an Aircraft Spruce M1000 CHT on the 914.

[Al Schober]

Perhaps if we monitored EGT in our A4s, we could get the black iron exhaust to last longer?? PLEASE take this as a joke!

[joe_db]

I data log with an Innovate Motorsports LM-2 and use an Aircraft Spruce M1000 CHT on the 914.

Are you talking Rotax 914? That engine has water-cooled heads IIRC.
Between that and no manual mixture control I am not sure what the CHT would be for Maybe early warning that the radiator popped a leak?

[yeahmag]

Porsche 914 (4 cylinder air cooled)

[joe_db]

Porsche 914 (4 cylinder air cooled)

This was my 914:
2.0 liter high compression piston/cylinder set, twin Dell'Orto downdraft carbs, balanced, 3 angle valve job, lightened flywheel, a big oil cooler, and a cam I cannot recall the numbers on. It got a rebuild at 100K, another around 200K, and burned up around 300k
It was a gasoline quality detector too, anything less than 93 octane had it complaining.
The Rotax 912 and 914 series engines are a weird hybrid, air cooled cylinders and water cooled heads.