Printable Version of Topic

Click here to view this topic in its original format

914World.com _ 914World Garage _ Car dies when AFM plugged in?

Posted by: brooks944 Nov 17 2015, 02:37 PM

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

Posted by: Dave_Darling Nov 17 2015, 03:47 PM

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

Posted by: brooks944 Nov 17 2015, 04:28 PM

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?

Posted by: Dave_Darling Nov 17 2015, 05:25 PM

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

Posted by: r_towle Nov 17 2015, 09:48 PM

Start reading

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

Posted by: brooks944 Nov 18 2015, 12:17 PM

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.

Posted by: brooks944 Nov 18 2015, 02:15 PM

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

Posted by: brooks944 Nov 18 2015, 06:24 PM

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

Appreciate any help.

Posted by: Dave_Darling Nov 18 2015, 06:35 PM

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

Posted by: brooks944 Nov 19 2015, 12:08 PM

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

Posted by: brooks944 Nov 19 2015, 12:56 PM

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?

Posted by: Dave_Darling Nov 20 2015, 01:09 AM

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

Posted by: r_towle Nov 20 2015, 07:00 AM

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

Posted by: poorsche914 Nov 20 2015, 09:10 AM

Rick,

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

steve

Posted by: timothy_nd28 Nov 20 2015, 09:46 AM

Let him keep guessing, he's all over the map. He didn't follow my directions and started a new thread

Posted by: timothy_nd28 Nov 20 2015, 10:54 AM

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.

Posted by: timothy_nd28 Nov 20 2015, 11:14 AM

Make sure your airflow meter isn't doing this

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

Posted by: brooks944 Nov 20 2015, 12:42 PM

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

Posted by: brooks944 Nov 20 2015, 12:54 PM

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

Appreciate the expertise,

Rick

Posted by: timothy_nd28 Nov 20 2015, 01:55 PM

Posted by: brooks944 Nov 20 2015, 02:47 PM

Thanks.

Between pin 7 and 8 reads 208 ohms
Between 6 and 9 reads 280
Between 27 and 6 nothing, my meter reads 1 when there is no connection so it stayed at 1
Between 13 to ground nothing stayed at 1

Outside temp is between 60 and 65, my garage isn't heated so that is representative. Look forward to your next bit of guidance. Thanks!!!

Rick

Posted by: r_towle Nov 20 2015, 02:59 PM

Listen to Tim.....nuff said

Posted by: brooks944 Nov 20 2015, 04:00 PM

Thanks, I am listening to Tim!

Posted by: timothy_nd28 Nov 20 2015, 04:51 PM

I'll assume reading "1" on the meter represents infinity resistance. Pin 13 is the cylinder head temperature sensor, this needs to be connected for your engine to run. Investigate the wiring to the CHT and recheck pin 13 to battery ground.

Posted by: brooks944 Nov 20 2015, 05:27 PM

Thank you Tim, I will check that first thing tomorrow and report back.

Posted by: brooks944 Nov 21 2015, 08:41 AM

Tim, the wiring to the CHT seems to be OK. When checking pin 13 to ground again no change on the meter on any setting. I connected the opposite connector from the meter to the battery ground wire and bare metal, is that
right?

Thanks,
Rick

Posted by: timothy_nd28 Nov 21 2015, 08:48 AM

Shouldn't matter, instead of probing battery ground, probe the engine tin. If no change, disconnect the CHT and put your meter lead on it directly with the other meter lead on ground.

Posted by: brooks944 Nov 21 2015, 09:20 AM

Tim, I checked between pin 13 and 17 and got 2004 ohms. It's 63 degrees in my garage right now. Does that mean we're good with the HT? On my meter, a reading of one means go to the next scale, so operator error! Look forward to your reply.

Rick

Posted by: timothy_nd28 Nov 21 2015, 11:47 AM

2k ohms between pins 13 and 17 is a good number for the CHT. Check pin 13 to 16 and see if you get the same number. Also check pin 13 to pin 5, should be the same.

Recheck pin 27 to pin 6, this is the temp sensor within the airflow meter

Posted by: brooks944 Nov 21 2015, 12:50 PM

The temperature outside is higher, about 65. Here are the numbers I got, are on the 20k scale of my meter:

13 pin to 17 - 0.28
13 to 16 - 0.29
13 to 5 - 0.29
27 to 6 - 1.82

Thanks Tim, I really appreciate the help.

Posted by: timothy_nd28 Nov 21 2015, 01:00 PM

pin 6 to pin 27 seems out of range, but not sure if that's the problem you are having.

Remove the black plastic cover on the AFM, and post a picture of the resistor plate. Run to radio shack and get a pack of 2k 1/2watt resistors

Posted by: timothy_nd28 Nov 21 2015, 01:07 PM

Also, grab a bag of those alligator clips with multi color wires

Posted by: brooks944 Nov 21 2015, 01:20 PM

Thanks, won't be able to get them until Monday, my wife says do the leaves thing...

And I have the local PCA autocross tomorrow, but will open up the AFM and add pictures. I do have two AFM's since I bought one with another used FI system and both were behaving the same. One is rebuilt, and that's the one in the car now. Does that help narrow the problem some?

Thanks again for the help.

Rick

Posted by: timothy_nd28 Nov 21 2015, 01:30 PM

Pin 27 to 6 reads 1.82 on the 20k range on both?

Posted by: brooks944 Nov 21 2015, 03:29 PM

It read earlier 1.82 on the 20k scale and 196 on the 200k scale. Now it's reading 1.04 on the 2m scale. Is my meter bad?

Thanks Tim,
Rick

Posted by: timothy_nd28 Nov 21 2015, 03:48 PM

That would be a good idea, buy a cheap or borrow from a friend an autoranging multimeter. Parts on our cars are pretty expensive, I would hate to replace anything that still works because it was misread on a meter

Posted by: brooks944 Nov 21 2015, 04:04 PM

Thanks, I'll go get one Monday.

Posted by: brooks944 Nov 24 2015, 01:08 PM

Tim here is a picture of the AFM, please let me know what you see and what to do next.

Thanks,
Rick