Hi folks – I’d really like to get the hot start issue on the 1975 taken care of and there are times when the hot start “trick” of simply using a quarter throttle pedal when cranking doesn’t work.

From doing a bunch of reading it looks like the CHT is a likely suspect and I’ve seen reference me to using a spacer between the engine and CHT specifically to help address this. Any been there done that experiences or recommendations to share?

Thanks!

there is a in-line resistor that can be added that helps with this,
you can get a variable one and install it temporarily to see what resistance fixes the issue and then put together one that is same or close resistance to install in line.

Phil

Before we start talking fixes . . . do you know what your current sensor is doing . . . have you measured it both hot and cold?

From an engineering perspective:

The CHT spacer is a band-aid and can only affect transient conditions like a very short cycle time between a hot shutdown and a short time until it is attempted to be restarted.

The CHT spacer only inserts a time lag. Metal is a great conductor of heat. All the spacer can do is slightly increase the time it takes for the CHT to respond to transient changes. Eventually the spacer will saturate and the temperature of the head and the spacer will become the same. Both on the heating cycle and on the cool down cycle.

I'll stop there and defer to others that have practical field experience with it.

If you haven't measured what you already have and know the CHT to be good, that should be done 1st.

What is happening exactly when you try to hot restart and what does it take to get it started?

What is the ambient temperature where you are driving?

Actually, i guess no one has asked but is this a 1.8 or 2.0?


Assuming its a 2.0, I suspect replacing the head temp sensor or a spacer wont get you much here unless you have actually confirmed it to be bad. Most/all hot restart issues i have seen have been due to the engine being lean when cranking and the failure of a CHT will most likely cause you to go rich as more resistance or a circuit break causes a richer condition when it comes to the CHT.

Most hot restart issues i have seen recently have been due to either quick bleed down of fuel rail pressure, or crap ethanol laden gas boiling in the fuel lines, or both.

What they said. Get some hot and cold resistance readings, then we can go from there.

Thanks for all of the responses, I am still very much in the early days of learning about these systems.

This is a 1975 Djet 2 L car, stock engine.

Overall symptom is reluctance to start when hot and it tends to stumble off idle when it’s been running for half an hour or more. Cold start when sitting overnight or having rested for 30 minutes or more after being run a lot is totally fine, so it’s a heat related transient issue.

When cold the car starts without any additional throttle and idle’s is very solid/stable. When the car is hot it takes a lot of cranking to fire, and generally needs quarter throttle open to start. If that doesn’t work I need to floor it and keep it cranking for quite a bit before it will catch.

@superhawk - thanks for the education on the spacer, I had thought that the distance would result in a small but big enough space to change the reading, that was obviously an incorrect on something on my part. Thanks for the better information.

To do the resistance readings just get an ohm meter on the sensor output tank and then run the throttle through range?

Have you ever tried cycling the key on, letting the pump run, and turning the key off again like 4-5 times before cranking when hot?

I suspect 1 of 2 things (or maybe both).

1. Fuel flow issue of some sort. dont normally see this as much with a front pump but ethanol gas seems to boil really easily in the fuel rails in hot climates.

2. its a 75 car in California so I assume you are emissions testing it? Possible that its tuned so lean in order to pass emissions that after it heat soaks from sitting its not getting enough fuel to easily start. Maybe try fattening up the idle mixture

Check your fuel pressures when hot.

You could also try adding potentiometer inline with the CHT sensor and dialing up the resistance slightly to see if it eliminates the off idle stumble.

Whatever the underlying cause it it sounds like its lean to me.

Just let it get nice and cold and put a multimeter on it. The cht is is near cylinder 3, passenger side, towards the rear of the car. Should be a little quick connect but on the wire. Then get the car nice and hot and check it again. Varies by year but it should head to 0-200 ohms hot 2k-4k cold. Malfunction here would result in a rich condition when hot.

Hopefully.

There could also be a cascade of issues that are not hard to fix but will take some sleuthing (fun) to figure out. I find that in these early fuel injection systems people meaning well will patch things in ways that end up causing stuff like what you are experiencing.
.

EmoryGT350 has it. really has nothing to do with throttle per se.

measure cold, measure hot.

here is the chart for -- there are variance in CHT resistance curves that vary by CHT part number but here are a couple of the common ones (p/n 017 and 012) to give you the general idea. One lead comes off the CHT. Use the engine case as ground then measure resistance.

Click to view attachment

Really solid inputs from these guys.
For my part, I know I have a fuel pump that loses all pressure as soon as it shuts down. So, I have to crank a bit too much for any start, and on real hot starts, its a challenge to get the car started again. The engine starts best when it's reasonably warm after running.

So, I'll not be chasing anything else until I get the fuel system sorted and holding pressure when it's shut down.

I still suspect pressure isn't going to be your thing Van. It comes up so fast on cranking... I get maybe 10 minutes before my pressure is 0.

I don't have huge confidence either. But you've seen that I've literally gone through EVERYTHING in the FI system. And, with 49yr old fuel lines staring me in the face every time I get under the car, I think it's due from that perspective alone. Then, as I resume trouble shooting, I won't need to consider it in my troubleshooting.
Getting the fuel pump off of the coil circuit made a notable difference in starting but didn't correct everything completely. So, I'm of the "cascade" mindset as you put it.

I was just down there and noticed one of my fuel lines coming off the pump was rubbing on the body. Always scary.

A hot, dirty distributor with 40 year old grease in between the advance plates can stick in the advanced position and make a hit car very hard to start.

I would suggest you clean the ignition system prior to testing the fuel injection system.

Rich

I forgot about that. Checking the movement of the advance plate is a great idea.

Hi guys - thanks for the coaching and background info.

I got infinite resistance at first because the tech who lasted worked on the car oversprayed the lead with black paint and there wasn't any connection.

After wire brushing the lead I got 2.35 at the 20K OHM setting at 70 degree outside temperature, engine sitting cold overnight.

About right cold. Reconnect, go drive car. Check again when hot.