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high cht reading

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dan carley

Well Known Member
in my rv4 I have a 0-320 which always ran about 350 degrees on number 3 cylinder. I had to replace the cht sender and now it's reading 400 degrees. could it possibly the sender? or should I be looking at something else

thanks in advance
dan caelry
 
Can you use a laser temperature gauge and see if it matches your CHT? (You will have to laser it right next to the temperature probe.)
 
N941WR said:
Can you use a laser temperature gauge and see if it matches your CHT? (You will have to laser it right next to the temperature probe.)
Click to expand...

Toobuilder said:
Swap senders from one cylinder to another and see if the temp follows
Click to expand...

I had to laugh when I saw Mike's response. I assumed he only had one CHT probe and Mike assumed he has four.

That said, I like Mike's suggestion.
 
Keep in mind that the CHT reading the probe sees is deep in the head, right on the roof of the combustion chamber. The head is going to be considerably cooler anywhere else and especially at the surface. If you're looking at an IR thermometer, not much chance you are going to get a strong correlation even if you aim it right on the probe body.
 
I have a similar problem on my RV-9A with #3 cylinder. In hot weather, not only the CHT but the EGT are excessively high. We've tried everything and then some...
 
As an FYI - there are certain 'basics' to look at before doing a lot of other troubleshooing here.
1. CHT - it's generally recognised that the baffling directly behind the #3 cyl needs to be stepped of the head are (to give a gap for the air) a search will find the answer.
2. EGT - one simple check is to see what the fuel flow is at max power takeoff (adjusted for or at sea level) it should be approx 13.5 gals for 160hp & approx 16 gals for 180hp engines - mine was 10gal/hr (160hp) until I found out what it needed to be, that caused higher CHT'S in climb. Being a carby version the needle needed to be a different size, on fuel injected engines the fuel control unit needs 'adjusting' by a shop. (my big mistake was thinking that the Lycoming factory new engine was just a drop in item - works for certified airframes but not for homebuilts as the baffling & air intake systems can be/are totally different & resulting in different airflows).
Once the above is sorted then if you still experience high temps you need to look further as to the cause.
 
jakej said:
2. EGT - one simple check is to see what the fuel flow is at max power takeoff (adjusted for or at sea level) it should be approx 13.5 gals for 160hp & approx 16 gals for 180hp engines -.
Click to expand...

This has a good amount of variability based upon your prop and the RPM's your turning. For example in my 320, I cruise climb at 140 MPH, which turns 2550 RPM with my FP. I lean a bit down to around 12.5 GPH (150-175 ROP). I lose a decent amount of power if I don't lean in my initial climb 100 MPH / 2440 RPM / 11.8 GPH. THe FI wants to throw 13.5-14 GPH at it and I lose at least 10% power those higher flows, not to mention the waste. My climb CHT's are under 400.

Do you have EI? Advanced timing will increase your CHTs. The only time I hit 400* is at the very end of a 10,000' climb. This is because the advance keeps increasing as the MAP drops with altitude and this drives up the CHTs even though I am under 75% power.

Larry
 
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I have to throttle back and increase airspeed to keep my CHT's out of the red (420) on all cylinders, where should start?
 
Do a search for high CHT's. There LOTS of info on here for that. The first thing is make sure all baffling is nice and tight. My experience.... I had high temps until i provided a pathway for airflow to get below the point where the cylinder walls and fins literally come together essentially blocking the airflow. I fixed that on number 2 and 3... temp problem solved. Do the search there are plenty of photos to see regarding this.
 
CHT RED LIBE

CHT red line for most normally aspirated Lycoming's is 500. Recommended high performance cruise limit 435. Recommended economy cruise limit 400.
Before the piston rings are fully seated CHT is a big concern. After the rings are seated not really a concern if kept below the above numbers.
I have around 2500 hours in Pitts Aircraft. About 99% of this was with no CHT installed. Never an engine problem.
 
dan carley said:
I used a later temperature tool and the cht is off by 55 degrees to high
I ordered a new probe
Click to expand...
Well that is good news. What brand or kind of probe (plug gasket or bayonet) and brand and type of gauge (electronic or mechanical). Ordering the wrong type (K or J junction) is a common mistake, even brands of probes can work differently. Probe measures the engine CHT and cold junction ambient temperature at gauge, which the gauge should compensate for. Gauges can be off, especially older analog ones.

Under 400F is ideal for long life. If you are at 400F at high power an/or climb you are in ball park.

jrs14855 said:
CHT red line for most normally aspirated Lycoming's is 500. Recommended high performance cruise limit 435. Recommended economy cruise limit 400.......About 99% of this was with no CHT installed. Never an engine problem.
Click to expand...
Yep in the day most planes had ZERO cht or egt gauges (certified or experimental). At most one EGT or one CHT was a big deal.... Remember renting planes, getting all excited when the plane had one EGT.... Now if you don't have EGT/CHT on every jug, FF, MPG, full EFIS... you are sad and embarrassed... Ha ha. However keeping it below 400F is goodness. 500F may be absolute red-line but that will eat your cylinders. Anything over 420F for me (hot day, heavy long climb) gets me nervous.
 
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jakej said:
As an FYI - there are certain 'basics' to look at before doing a lot of other troubleshooing here.
1. CHT - it's generally recognised that the baffling directly behind the #3 cyl needs to be stepped of the head are (to give a gap for the air) a search will find the answer.
2. EGT - one simple check is to see what the fuel flow is at max power takeoff (adjusted for or at sea level) it should be approx 13.5 gals for 160hp & approx 16 gals for 180hp engines - mine was 10gal/hr (160hp) until I found out what it needed to be, that caused higher CHT'S in climb. Being a carby version the needle needed to be a different size, on fuel injected engines the fuel control unit needs 'adjusting' by a shop. (my big mistake was thinking that the Lycoming factory new engine was just a drop in item - works for certified airframes but not for homebuilts as the baffling & air intake systems can be/are totally different & resulting in different airflows).
Once the above is sorted then if you still experience high temps you need to look further as to the cause.
Click to expand...

Jake,

That should be about 15.5 for the 160HP and about 17.5 for the 180 HP ;)
 
Ok, I got the incorrect numbers out and the litre to gal conversion - it’s been nearly 2 years, were you asleep :D
 
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