3.2 Head Temperature Sensor curiosity

[Another911]

My '85 Coupe wouldn't start in cold weather (under 40F). I tested for spark - good. I could smell gas. I assumed the Idle Control Valve controlled air during start-up so I checked it - good. Then checked the Head Temperature Sensor (CHT) using the paperclip bypass method - - bingo, started right up. The CHT had already been changed with a 2-wire, but it looks like that failed as well.

Question - could someone explain what the CHT does to affect fuel/air mixture for cold starts. Knowing how to fix stuff is a lot more gratifying when I understand what it is I did. Thank you in advance!

[Tippy]

Originally Posted by Another911

My '85 Coupe wouldn't start in cold weather. I tested for spark - good. I could smell gas. I assumed the Idle Control Valve controlled air during start-up so I checked it - good. Then checked the Head Temperature Sensor using the paperclip bypass method - - bingo, started right up.

Question - could someone explain what the Head Temperature Sensor does to affect fuel/air mixture for cold starts. Knowing how to fix stuff is a lot more gratifying when I understand what it is I did. Thank you in advance!

The CHT tells the computer when it is cold that there needs to be additional fuel injected during warmup. I believe the cutoff is 155F and the engine is considered warm.

[mysocal911]

Originally Posted by Another911

My '85 Coupe wouldn't start in cold weather. I tested for spark - good. I could smell gas. I assumed the Idle Control Valve controlled air during start-up so I checked it - good. Then checked the Head Temperature Sensor using the paperclip bypass method - - bingo, started right up.

Question - could someone explain what the Head Temperature Sensor does to affect fuel/air mixture for cold starts. Knowing how to fix stuff is a lot more gratifying when I understand what it is I did. Thank you in advance!

Most likely you have a bad temp sensor (open). If it's a signal wire one, it needs to be upgraded to a two wire.

[917_Langheck]

Originally Posted by Another911

The CHT had already been changed with a 2-wire, but it looks like that failed as well.

Always refer to the source... :-)

[mysocal911]

Originally Posted by 917_Langheck

Always refer to the source... :-)

The OP later added the two wire sensor info after two posts.

[wazzz]

I probably misunderstand it, but since the CHT is a NTC sensor, substituting a short (paperclip or whatever) would trick the ECM into thinking the engine is warm, right?
So I wonder how that trick helped the OP with cold start under 40F ambient?

[mysocal911]

Originally Posted by wazzz

I probably misunderstand it, but since the CHT is a NTC sensor, substituting a short (paperclip or whatever) would trick the ECM into thinking the engine is warm, right?

Yes, keep one in your glovebox.

Originally Posted by wazzz

So I wonder how that trick helped the OP with cold start under 40F ambient?

The engine was most likely flooded even at 40F, so shorting the sensor allowed the engine to start sooner (leaned mixture).

[wazzz]

Got it. Thanks!

[917_Langheck]

Originally Posted by mysocal911

The OP later added the two wire sensor info after two posts.

Well, that's just not cricket!

[Mr. Merk]

Mine failed a couple years ago when I was 9 hours from home on the way back from Luft Indy. Luckily it was already warm, and I was able to ground the connector to the intake manifold and drive home without issue. (we ripped some horn wiring out of my buddy's 930 to make a jumper)

It did NOT want to cold start though. It took forever to get it started.

[mysocal911]

Originally Posted by Mr. Merk

Mine failed a couple years ago when I was 9 hours from home on the way back from Luft Indy. Luckily it was already warm, and I was able to ground the connector to the intake manifold and drive home without issue. (we ripped some horn wiring out of my buddy's 930 to make a jumper)

It did NOT want to cold start though. It took forever to get it started.

You forgot your paper clip in the glovebox, right? For cold starting, you can remove the paperclip for a one second crank, and re-install.

[Mr. Merk]

Originally Posted by mysocal911

You forgot your paper clip in the glovebox, right? For cold starting, you can remove the paperclip for a one second crank, and re-install.

Ha, I wish I knew that trick at the time.

Jumper both terminals together for cold start, and ground to intake to run/hot start.

[mysocal911]

Originally Posted by Mr. Merk

Ha, I wish I knew that trick at the time.

Jumper both terminals together for cold start, and ground to intake to run/hot start.

Hot start & running - just jumper two pins on female connector
Cold start - momentarily (few seconds) remove jumper while cranking

[theiceman]

Originally Posted by Mr. Merk

Ha, I wish I knew that trick at the time.

Jumper both terminals together for cold start, and ground to intake to run/hot start.

no .. the reason it worked for him is because his particular issue was a flooded engine. The HTS was not the route cause of his issue, but shorting the sensor with the paperclip leaned it out helping it start in this particular situation.

The issue still remains of why it flooded and did not start in the first place.

[Mr. Merk]

When my sensor failed, it flooded and raw gas was emitting from the tail pipe.

[mysocal911]

Originally Posted by theiceman

no .. the reason it worked for him is because his particular issue was a flooded engine. The HTS was not the route cause of his issue, but shorting the sensor with the paperclip leaned it out helping it start in this particular situation.

The issue still remains of why it flooded and did not start in the first place.

It appears, per the OP's statement, that the replaced 2 wire sensor failed in the open state causing a flooded engine.