Car dies when AFM plugged in?, 1975 1.8 with L-jet Options

[brooks944]

Hi, looking for some suggestions on what my issue is. My car won't start with the AFM connection plugged in but will with it out. Once started, if I plug the connector it the idle goes up then the car dies. Once warmed up I can plug it in and it will run and start while connected, but cuts out and backfires and almost dies. Seems to run better when connected before the cut out/backfiring/almost dying. Any suggestions?

Always appreciate the help!

Rick

[Dave_Darling]

The fuel pump shouldn't even run if the AFM is unplugged, once the starter is no longer cranking. Can you verify that?

Fuel pressure? Head temp sensor plugged in? Anything else not plugged in securely?

--DD

[brooks944]

Thanks Dave. The fuel pump has been rewired on my car so it comes on with the key, I know that's wrong for a 75, but that's what I got from the last owner. Don't know where the head temp sensor is supposed to plug in, can you advise?

[Dave_Darling]

It's in the head near the #3 spark plug; should plug into the FI wiring harness with a single-wire plastic box connector.

--DD

[r_towle]

Start reading

https://www.type4.org/manuals/ljet/

[brooks944]

Thanks Dave found the head temp sensor, it is connected to the harness, single wire but it's a metal connector. Can't find anything else electrical that isn't connected. What do I need to do next?

I am researching the link from the next reply as well, thanks to both of you.

[brooks944]

I just noticed I have the adjustable fuel pressure installed and read it wasn't for the 75 l-jet cars. Could that be the problem? I do have another used one that is not adjustable, I can put it in but don't know if that one is good or not. Should I try it?

Thanks,
Rick

[brooks944]

Sorry, think faster than I type. It' a fuel pressure REGULATOR.

Appreciate any help.

[Dave_Darling]

The ones for the D-jet cars are adjustable, but not vacuum-referenced.

The ones for the L-jet cars are non-adjustable, but are referenced to manifold vacuum. There will be a hose from the manifold to the FPR if you have this type.

There are aftermarket ones that are adjustable, vacuum-referenced, and have bells and whistles too.

I don't remember exactly what kind of signal the AFM sends. It may be a resistance--and if so, unplugged would look like infinite resistance. Which might be the meter all the way open (add as much fuel as possible) or all the way closed (add as little fuel as possible).

A 75 would have the air temp sensor in the AFM as well, and having that unplugged would look like very very cold air (add more fuel). You might try getting a box of resistors to see if you can fake various signals to the FI (what the AFM should be reading) and see if any one particular connection is what kills it.

--DD

[brooks944]

Yes, my FPR has one connection that runs to the cold start valve in the intake and another that runs across the back of the engine bay then disappears into the body below the battery. This is different than references I've seen for 74 l-jet, since my car doesn't seem to have the decel valve connected or a charcoal canister.

With the AFM unplugged, it seems to be running rich (but running) as you suggested if might be. To test with resisters would you run an individual one from the connector to each male connector on the AFM? Sorry for the dumb questions but electrical testing is not what I am use to.

Appreciate the help,
Rick

[brooks944]

Correction, my FPR is only connected to fuel lines, there are no vacuum connections on it. Should I put the other FPR in that I have and run the vacuum connection to the manifold like the diagrams show?

Is vacuum even an issue (or fuel for that matter) since the can runs with the AFM disconnected and will run with it connected after warming up, although it will backfire and cutout?

[Dave_Darling]

The injector flow should be determined by the size of the injector opening, and the difference in pressure between the fuel on the one side of the injector, and the air on the other (manifold) side.

In a D-jet car (and in yours), that pressure difference changes depending on throttle position and RPM. In a stock L-jet setup, the fuel pressure is referenced to the manifold air pressure, so the difference is constant.

At higher throttle settings, you'll be getting less fuel than your FI thinks you should be. That is usually a Bad Thing, but with the way your system is working it may not be.


As for the resistors, I'm not completely sure. There should be a few circuits in the AFM; one of them just connects two wires when the flap opens. That one runs the fuel pump. The 75 should have the air temp sensor in it, so there will be one contact that changes resistance to the ground contact (might be its own ground contact specifically?) with temperature. There should be at least two more that vary resistance with the movement of the flap in the meter.

Some Google searching may turn up what the specific contacts in the meter do. Or, if you're careful, you can pry the cover off the electrical part of the meter and inspect it.

I'm starting to think that you may need to find all of the non-stock bits of your system and return them to stock. That may make troubleshooting easier...

--DD

[r_towle]

You could read the link I posted for testing ljet, or keep guessing

[poorsche914]

Rick,

Is this still a 1.8 or is it a 2.2? (IMG:style_emoticons/default/confused24.gif)
From the previous owner's signature: 1975 Porsche 914 2.2 / Scarlet Red (L31M) aka "Scarlet" (Sold)

steve
(IMG:style_emoticons/default/driving.gif)

[timothy_nd28]

Let him keep guessing, he's all over the map. He didn't follow my directions and started a new thread (IMG:style_emoticons/default/bs.gif)

[timothy_nd28]

My apologies! I've been helping a handful of people this month with problems of their Ljet systems, and I mixed up who is who.
Your problem seems awfully similar to Blue6's issue. I have been helping people through texting, PM's, and posting on their threads, and got overwhelmed.
Since I made an ass out of myself, I'll give you some guidance on how to proceed.

Make sure the airflow meter's connector is connected. At the ECU (under battery), remove the big connector.
With your voltmeter, check the resistance between pin 7 and pin 8, should be between 100-200 ohms

Then, check the resistance between pin 6 and pin 9, should be between 200-400 ohms

Next, check the resistance between pin 27 and pin 6. Report this resistance along with the current outdoor temperature.

You should also make sure the flap inside the airflow meter does move freely, and doesn't hang up.

Since you will be doing the resistance test at the ECU plug, go ahead and check pin 13 to battery ground. This is the CHT sensor, be nice to know what that's reading.

[timothy_nd28]

Make sure your airflow meter isn't doing this

https://www.youtube.com/watch?v=-4nDzPI_kZ4

[brooks944]

Steve, yes the PO said 2.2 but when I posted the engine rebuild specs here in a post about a year ago I was informed it's a 2056. The receipt I have says 'rebuild to 2L.

Excited to see you car with the new engine, impressive numbers! Thanks for the reply,
Rick

[brooks944]

The AFM is moving smoothly. Do I need to disconnect the battery to check the resistance at the ECU?

Appreciate the expertise,

Rick

[timothy_nd28]

No