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Larry
Larry, Automotive technical expert
Category: Car
Satisfied Customers: 1139
Experience:  30 years automotive technical writer, editor & author
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bad miss when engine is under load, and HORRIBLE gas mileage.(5 mpg)

Customer Question

1992 k3500 engine light is on I. I have a bad miss when engine is under load, and HORRIBLE gas mileage.(5 mpg) The truck has a 7.4L 454. Replaced plugs wires, cap rotor and checked compression, leak down test ect. I convinced it is electrical. So I checked the timming. I also checked the engine codes. Code 43 came up. I replaced the knock sensor. still miss and code. I checked timming with ecm wire pulled. It is at 4 btdc. With wire installed at idle, 16 btdc. I''m not sure if it is related but the oil pressure is some times ok 30-40 other times it is maxed out, pegged to the max.
Submitted: 6 years ago.
Category: Car
Expert:  Larry replied 6 years ago.
I doubt the knock sensor code/EST timing is a factor with the misfire. Your timign is right on spec (4 degrees at idle w/o EST).

A misfire usually indicates one of three things: ignition misfire due to a bad spark plug, bad plug wire or weak coil, lean misfire due to low fuel pressure or a vacuum leak, or compression misfire due to a leaky valve or weak valve spring.

You say you have checked compression, and is it okay, so it is unlikely the engine has a burned exhaust valve or blown head gasket. But it might have one or more weak valve springs, especially if it has a lot of miles on it.

You say you have replaced the plugs and wires. You could still have a weak coil.

As for the fuel system, the engine could have low fuel pressure (you should have 9 to 13 psi with the key on and engine off), or a dirty fuel filter or dirty injectors.

I'd also check the engine carefully for any leaky vacuum hoses, intake manifold or throttle body gasket leaks, and check the vacuum connections at the PCV valve and EGR valve.

Customer: replied 6 years ago.
What I don't understand is this Miss is only when I'm pulling a hill under load. It will get to the point where The transmishion will down shift. when the RPM's go up it helps some but it continues to miss slighlty at the higer rpm's till the top of the hill. If I'm going up hill with a slight grade in O/D you can fell it missing/hesitating. Almost like it is loading up. I've even had it backfire or what seemed like it.

No load the truck should have plenty of power but dosn't. I also don't understand why the coil or fuel problem would effect my fuel consomption so much. I went from 12mpg to 5. I was under the understanding with this knock sensor being tripped it would go into a "Limp mode" that would retard the timming to some safe setting. I was thinking that what might be causing the hesitation/fuel consumption. Also How can I comfirm it is the coil? thanks
Expert:  Larry replied 6 years ago.
The reason for the drop in fuel mileage is the misfire. Many mnisfires you can't really hear or feel -- unless the engine is working hard under load (as when driving up a hill when it becomes more noticeable).

To check the coil, pull a plug wire off of a spark plug and place it near a metal surface on the engine. Then start the engine. You should see a spark jump from the plug wire to the engine and hear it snap. No spark would indicate a weak coil.

You can also check the coil's resistance with an ohmmeter. Here's the procedure for checking the coil:

Make sure the ignition switch is "OFF".

1. Disconnect the distributor lead and wiring from the ignition coil.
2. Connect a suitable ohmmeter as shown in step 1. Use the high scale. The reading should be infinite. If not, replace coil.
3. Connect ohmmeter as shown in step 2. Use the low scale. The reading should be very low or zero. If not, replace coil.
4. Connect ohmmeter as shown in step 3. Use the high scale. The meter should not read infinite. If it does, replace coil.
5. Reconnect the distributor lead and wiring from the ignition coil.

graphic
Customer: replied 6 years ago.
I checked the coil and the fuel pressure both are good.Even though I put new plugs in the turck, I pulled each one to make sure every thing was working as it should upon your sugestion on problems with spark. ONE PLUG WAS NOT!. I can't believe it, but a brand new plug wasn't even working. I replaced it and it seems that the miss went away. I did notice that the brand new boots on two cylinders are falling apart where the boot slides over the actual connection point on the plugs. THEY ARE ONLY 1 MONTH OLD. Could this be Arching? Any Idea why?
Also do you know what color wire goes from the knock sensor to the ECM? I think that may be shorted where it goes past the exhaust header.

Anyhow,... I reall appreciate your help. I would never have guessed that the knock sensor code/engine light, oil pressure acting funny and the miss were not connected. I just assumed I had some electricl control issue with the ecm or knock sensor creating the miss. Im going to change the oil pressure switch after I do a manual pressure check.
    And the knock sensor I'll post another paying question if I can't figure it out.
Thanks again. I will release payment regaurdless of reply ..Thanks alot!!!
Expert:  Larry replied 6 years ago.
I'm curious, what was the brand of the bad spark plug?

As for the plug boots falling off, I would not think that would be an issue with a quality name brand plug wire set (Delco or Bosch). But if it is a no-name brand I would not be surprised. If they use a cheap grade of synthetic rubber for the boots, the boots will harden and lose their elasticity from exposure to heat. It is a quality issue, not arcing.

As for the knock sensor, the wire should be dark blue.

graphic

Here's a diagnostic chart to help you further troubleshoot the knock sensor also:

graphic


If I have helped you figure out your problems, please ACCEPT now. Thanks.




Larry, Automotive technical expert
Category: Car
Satisfied Customers: 1139
Experience: 30 years automotive technical writer, editor & author
Larry and 15 other Car Specialists are ready to help you
Customer: replied 6 years ago.
Ac delco plugs and wires They are soft, so soft they are like playdoo almost. thanks again

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