Help needed with diagnosis

[Pilgrim]

The car: 1983 280ZX Turbo, 120K miles, injectors, fuel filter all known good.

The car runs well EXCEPT that more and more frequently either at idle or when I'm driving and holding a steady speed, the revs drop just like I had pulled my foot off the gas. If this happens at idle, the car stalls. If it happens while driving, the revs keep dropping unless I hit the gas.

But - if I touch the gas pedal, the revs pick up and the car runs fine for another 5-10 seconds, then the power falls off again.

I can drive for extended periods with this going on, and as long as I push the gas pedal down when the power falls off, the power returns and it's OK for another 5-10 seconds, when the cycle repeats. The timing is very consistent, and it's clearly something causing the fuel system to stop delivering gas even though the gas pedal is depressed. But moving the gas pedal restores the fuel feed.

I thought this must be an ECU problem and found another ECU for the car - replaced it - but the problem continues. After looking through the shop manual, I see that the head temp sensor and the throttle position sensor combine to reduce gas flow when decelerating. My best guess so far is that one of the two is failing and sending erroneous signals to the ECU.

I replaced the throttle position sensor a couple of years ago, so I've ordered a replacement head temp sensor...hasn't arrived yet.

Does anyone have any other ideas as to what might cause this repeat cycle of power loss which is always corrected by hitting the gas pedal? :bulb:

Thanks.....

Edited November 30, 2009 by Pilgrim

[Neumeier]

I think it is your air flow meter (AFM). This is one of the most important parts of the FI system, and the only one I can think of that will be affected by depressing the throttle. By measuring the amount of air entering the motor, it determines how much fuel the injectors send to the motor. If it sends faulty signals to the ECU, the car runs poorly. Basically, the air flap in the AFM is connected to a metal probe that swipes across a printed circuit board. This leads to a variable resistance that the ECU uses to determine the amount of fuel to send to the motor. The circuit board can get worn or dirty, and this has a major affect on how the car runs. The AFM also has a temperature probe, another resistor, that is used to correct for air density. Might be good to ask other members where to get an AFM. They are probably not cheap. Most parts suppliers (NAPA etc.) would sell replacements.

Good Luck!

[Pilgrim]

That may be right. I'll check the service manual to see if there's a way to test the AFM. You're right, not a part you want to replace unless you have to.

Do you by chance know of a test procedure?

[Neumeier]

The procedure will be testing the resistances of the AFM across its, usually, four terminals. This amounts to just checking whether or not they fall within the spec ranges. It may not tell you whether or not it is faulty during real operation. One other thing that could be important is checking the connector to make sure the contacts are clean. As cars get older, these contact issues become more of a problem. The connector could be loose as well.

It would be good to get your hands on the fuel injection service manual. I have one in pdf format for my '78, but your system is probably different. You could do a search on the web, or on this club site.

Also, you might just check at a few of the local parts dealers. AFMs are a pretty standard replacement part. They might be cheaper than I think.

[Pilgrim]

Sounds well worth checking! Thanks.

[Bcalvosa]

If power drops, it has to be fuel or spark. When cruising and power suddenly drops, if the tachometer needle immediately dumps to zero, it is likely ignition (since the tach is triggered by the ignition pulse). If the tachometer keeps indicating (slowly dropping in RPM), it is likely fuel. Fuel could mean Air Flow Meter (as mentioned), loose connections, etc. Ignition cutting out is often caused by the pickup for the ZX Turbo optical distributor.

[kenz240z]

A couple of thoughts..you might want to check the fuel filter and also check that none of the fuel hoses have cracks that could allow air to be sucked in.

[Pilgrim]

Good thoughts here, and I appreciate them. Fuel filter is new, but I could easily replace some hoses. The tach does keep indicating RPM so the problem is definitely fuel. I'm sure something is telling the fuel system to cut off - until I hit the throttle again. That inclines me to think that it is most likely an electronic signal to the fuel system, as the cutoff is immediate - and so is the recovery. It happens both at idle and when driving.

And we have gotten 6" of snow since last evening and another inch in the past hour....the ZX is sitting under a car cover in the driveway. Won't be able to reach it today!

[Bcalvosa]

If moving the throttle consistently makes it come back, it points to something that would react with the throttle movement (the throttle position switch or the air flow meter). If it is not repeatable, and just time makes it come back, relays or electronic circuits are more likely. If vibration, bumps, etc., are the cause and solution, loose connectors are more likely.

To eliminate the fuel pump, lines, filter, etc., you could tee a fuel pressure gauge into the fuel line, snake the hose through the hood vent, tape it to the outside of the windshield, and drive around waiting for it to cut out. Fuel pressure should fluctuate with boost pressure and manifold vacuum, but shouldn't die. If it doesn't die, you are looking for something that would interrupt the injector pulses...

[Neumeier]

It is probably not the throttle position switch, but could be worth checking anyway. It is used to tell the ECU one of two things: 1. It the throttle closed so that the ECU knows it should be in idle mode and 2. If the throttle is full open, to tell the ECU to provide a richened mixture. So, I don't think it would do anything when you are mid throttle, unless it had faulty contacts that were giving intermittent readings. So you could check that as well, just to be sure.

Another thing to do would be to inspect all vacuum hoses for cracks and leaks. In particular, the large rubber ACM to manifold hose is VERY important. If it has cracks in it, the AFM will not sense all of the air coming into the motor. THis can cause the motor to run lean and stumble.

[tdarone]

Pilgrim, did you ever solve this? I have a similar problem and was wondering if and how you fixed it.

Tom

Fairfax, VA

[Pilgrim]

Sorry, I haven't - the car has been under a cover and under a snowbank since I posted it. I'm going to have to wait for warmer weather, so I'm thinking March or April before I can get back to it. Based on all the feedback from this and another Z forum, I'm thinking air flow meter.