HELP! Surging, Hesitation, Backfire on EFI

[KDMatt]

Out of the blue today, the Z began giving me woes. I was on the freeway, I had just shifted into fifth, and I could all of a sudden feel what I can only describe as 'surges' the car would be pulling, and then it would be as though it's dying, and then pulling strong again, and then dying.

I quickly got off the freeway, and on the exit, back in first gear, I could feel the same thing more noticeably. I took another turn, and then all of a sudden it was fine for a while.

It started doing it again at a stop, I could see the tach bouncing up and down.

At home I popped the hood and started pushing on the throttle linkage a bit, if I barely pushed it in, the RPMs would go down, almost to the point of killing the engine.

I would push in the throttle a bit more, and it would take the engine a few seconds to rev up... and even with me holding in the throttle at a consistent opening, it would still surge.

To top it off, while I was doing this I somehow managed to get a backfire from the intake.

So... starved for fuel maybe? ... My worst fear = bad fuel pump

Somebody help me please! I've got a show on Saturday

[280~Master]

1st question is your fuel tank clean of all rust and crap? If it is then it could be your fuel pump, it may be something else though. Check your fuel presure first off. If thats low then I myself would pull the fuel pump out and clean the intake portion of it. There is a little filter on the intake/inside of your fuel pump. (just remember these original pumps can be ceased very easy from rust) My filter was removed when I replaced my fuel pump. Autozone will have a replacement in stock. Its a universal type, if you want to keep the Bosch style pump then you will most likely have to order it.

Once you have cleaned your pump up from crap then blow out your fuel lines with compressed air and replace your fuel filter if its dirty. I would also invest in an inline filter that sits between the gas tank and the fuel pump. For this you will need at least a 1 foot of 1/2 inch hose, 2 hose clamps and a 1/2 inch inline filter from NAPA auto parts. This is usally in stock at NAPA and I could not find one from other auto parts stores. Just ask for 1/2 inch inlet and outlet. its a napa gold part. I have lost the part number I was using.

[KDMatt]

Thanks for the suggestion master, it certainly seems like you might be right on about filter related stuff.

I let the car sit for over an hour while I did circuit tests on the ECU connector, and when I started it up again, it was fine.

So, maybe crud is getting trapped at the inlet for the pump.

I will look in this!

Also, the Fuel-Pump Killswitch in the AFM does not operate. This was the only circuit test for "surging" that my car failed (pp. 53 in EFI Bible). It still read continuity even with the AFM at rest.

[Grim]

If the fuel pump is fine look for air leaks in the rubber boot between the airflow meter and the throttle body. This can couse rough running and backfires in the intake.

[sblake01]

Check your temperature sensor as well as the connection to it. I've experienced the problem you described in both cars. On the Z it was a bad connection and on the 810 it was a bad sensor.

[KDMatt]

I tested the temp. sensor on the ECU harness, it read approx. 600 ohms when the air-temp sensor was about 1100 ohms, is that normal.

I was going to replace the temp sensor anyway, but I didn't realize a bad one could cause these symptoms. Thanks for the heads up.

[Rockr69]

If your car was running fine then started surging and backfiring, go with the Master's advice 1st! I've been through it all with my 76. The problem is the fine brass screen in the tank on the feed pipe to the pump gets clogged with varnish and fine rust particulates and since I don't have the dough to get my tank cleaned at radiator shop I got a 1/4" np tap and tapped the drain plug hole to accept 90 deg brass nipple. About 12" of fuel line, doesn't need to be

for fuel injection ( no pressure-just suction), plugged in a fram G3 fuel filter between pump and tank after cleaning inlet filter in pump and she pulls strong all the way to 6500. If there is a bunch of crap in your tank get 3 or 4 filters and keep swapping them out till all the crap is gone. You can clean them, but only about 3 times before they're totally junk. And don't worry about ruining your drain plug hole with the tap. If you don't drill it and just run the tap up, go slow backing up often, it will not ruin the stock threads and when you think you've got all the crud out you can thread the drain plug right back in. However take it from me if you've got the time and money drop that tank and take to radiator shop and have cleaned cuz I just changed another filter today and it's starting to get tiring. Hope this helps. Dan

[KDMatt]

Wait, there's a drain plug on the gas tank? ...

Or are you referring to the 'outlet' to the fuel pump?

[sblake01]

I tested the temp. sensor on the ECU harness, it read approx. 600 ohms when the air-temp sensor was about 1100 ohms, is that normal.

I was going to replace the temp sensor anyway, but I didn't realize a bad one could cause these symptoms. Thanks for the heads up.

Here's the test procedure for the temp sensor from the 76 FSM:

[Rockr69]

There is a drain plug on the tank, passenger side bottom.

[KDMatt]

Okay, so I did it today.

4pm to about 11pm with a dinner break and a parts run thrown in there.

I replaced the fuel filter in the engine bay as well as the associated hoses.

I also replaced the inlet hose at the fuel pump, and added a small filter between the tank and the pump, at Master's suggestion.

I noted a couple of things. One, when I disconnected the hose at the top of the fuel filter, there was barely any pressure, I'd say no more than a tablespoon's worth of gasoline came out, which I understand is not normal or good, a lot of gas came out the bottom though...

On the plus side, the gas I drained out of the tank was pretty clean. I didn't notice any rust coloration at all, just lots of clear gas. There were probably a few little particles of dirt in there, but nothing major.

After I reconnected everything and fired it up it seemed okay. I haven't noticed the surging or hesitation issue, but then again it happened sort of randomly that one day, so we'll see.

The idle speed seems to have dropped a bit, I dialed it back out with the bypass screw, and it seems to be idling pretty strong between 700 to 800 rpm (it idles low when it's 'cold' ... which I'm assuming is an issue with the Aux. Air Valve)

The one thing I have noticed is that the fuel pump is a lot quieter[i/]. I used to be able to hear it pretty distinctly, but now it's harder to make out, and I'm taking this as a good sign, maybe it isn't working quite as hard now?

Either way, this car was due to have its fuel filter changed (I'll upload some pics of my handiwork), the old one, now that it's out, looks like crap.

What's a good way to test the fuel pressure? I have a small hunch that maybe it's dropped a bit with that extra filter on the system now... The lady at Advance said it might take a few days for the engine to get 'used to' another filter, which I don't particularly buy, but what do you guys think?

[sblake01]

What's a good way to test the fuel pressure? I have a small hunch that maybe it's dropped a bit with that extra filter on the system now... The lady at Advance said it might take a few days for the engine to get 'used to' another filter, which I don't particularly buy, but what do you guys think?

I use a guage between the fuel fiter and the fuel rail. I bought a 0-100 psi pressure guage from the hardware store, some fuel line, a brass tee, and a couple of hose barbs. I just use mine for testing but I've seen some people leave them on the car. As far as the car getting used to the filter, well that comment doesn't make much sense since pressure regulator is on the fuel rail. Even if it dropped a few psi back at the tank/pump, it would still be high enough to to regulated to the roughly 37 psi at the rail. Did you test the temp sensor as per my post (#9)?