L-Jet not idling when HOT, Doesn't like 32C / 90F outside temps Options

[nihil44]

My '74 1.8 normally runs beautifully but with our present 90 degree F ambient temps it runs hot and won't idle.

Temp gauge is just short of the red zone. (Next time I will get NHS to put numbers on the gauges.)

Thermostat works - checked with hot air gun. Flaps are opening

Warm air guides installed

Tried enriching idle mixture by screwing in air bypass screw to max - no effect

Would appreciate help from the brains trust

Thanks

David
Brisbane, Australia

[falcor75]

Check that your fuel pressure still is ok, if the pump dislike the hot weather and cant keep the pressure up the engine might run leaner than normal.

[malcolm2]

I assume you are talking about oil temp. I also assume your temp gauge is the OEM set up at the Taco Plate? There are 4 or 5 different gauges, none have numbers. You can tell the difference by where the RED is. Just curious. You might try to find another way to check the oil temp. Do you have a Laser temp gun?

2ndly it might not have anything to do with temp. Maybe you have a hose off or leaking.

I recently took a 4 hour drive to Memphis. On the way home, as we exited the interstate, I noticed that my 1.8 L-jet was struggling to idle. Once home I found a hose that had split.

L-Jet does not like vacuum leaks and struggles to idle. Not always easy to find cause there are hoses and seals everywhere. As one member suggested recently, start at the intake and work your way to the air filter. At least you would have a system that way.

[nihil44]

I haven't checked fuel pressure but I have relocated fuel pump to the front using later model mounting. I expect the temp of the pump is not an issue
I have a fuel pressure gauge installed and will check this . Thanks

The car starts and idles no problem at normal temp but won't idle when oil temp climbs just short of the red. That is, only when it is quite hot.

It is a 911 style gauge using standard taco plate mounted temp sender. NHS restored the gauge and I sent them the sender to calibrate the gauge.

Thanks

[djway]

In the AFM box there is an air density/temp sensor that looks like a little cone. The metal contacts in there can get corroded which changes the resistance and throws the mixture off. When temps increase the resistance problem increases.
I fully agree that all the other things should be carefully checked but this is an item often overlooked.
good luck

[nihil44]

Thanks djway,

That diagnosis fits the scenario best. I will do some digging and report back

Thanks again

David

[nihil44]

Got an old AFM and looked inside but couldn't find the cone shaped sensor.

Could you please help identify it.

[nihil44]

Temp Gauge

[djway]

Got an old AFM and looked inside but couldn't find the cone shaped sensor.

Could you please help identify it.

Look inside the air intake end of the assembly. Some I have seen are cone shape and others are cylinder shaped. They have all been an off white plastic

[nihil44]

Got it.

Thanks

Will check on the car.
I am a little suspicious of the CHT sensor. Will also check that after a hot run

Thanks again

[djway]

Got it.

Thanks

Will check on the car.
I am a little suspicious of the CHT sensor. Will also check that after a hot run

Thanks again

This may help

Bosch L-Jetronic Fuel Injection System Basics
Ambient Temperature Sensor (S5) (aka. air intake temperature)
Function: Measures intake air temperature (air density).
Location & Operation: In the air meter, a NTC thermistor (temperature sensitive resistor) sends a signal to the injection ECU. Air density and air flow are used to calculate air mass and control fuel injection duration. Cooler air is denser, and requires a richer mixture than warm air.
Failure: Causes poor fuel mileage.
Test: For resistance rise & voltage drop at ECU pin 27 (ARM 04-23) when using a hot air blower versus ambient temperature (cold=high voltage, low ohms / hot=low voltage, high ohms / @20°C = 2.6 kohms, 2.6 VDC). Or use an ohmmeter to measure the resistance across the two pins of the temperature sensor.

Temperature F° Resistance Ohms
14 7,000-12,000
68 2,000-3,000
122 700-1,000
176 200-400
185+ Less than 200

Temperature Sensor
Temperature sensor resistance
The resistance between ECU plug terminals 13 and 17 (ground) should read 2-3 kOhms at 68 F/20 C. At 14 F/-10 C 7-12 kOhms, at 176 F/80 C 250-400 Ohms.
• Meter reads infinite resistance: Check temperature sensor directly at terminals on sensor. If sensor resistance is ok, replace cables. If tester shows 0 Ohms (continuity) at sensor terminals, replace sensor.
• Meter reads 0 Ohm (continuity): Pull plug from sensor. If meter at ECU plug then reads infinite, replace sensor. If not, replace cables.

[Valy]

Please check that the auxiliary air valve closes when the engine is hot. It's known to make trouble on Ljet. Try to plug the tubes that go to it after the engine wormed up to see if it's better.
And check for vacuum leaks again.

[nihil44]

Looks like it's fixed

A very talented friend of mine identified a loose oil filler cap as an air leak. Tightening the cap lifted the idle and it just needed a little back off on the idle air bypass screw and it is idling quite sweetly

Many thanks for all the advice offered