Backfire thru the throttle body

Under acceleration from stop, and under pressure as in going up hill. Loss of power also. I believe this to be a lean condition. Was wondering if it could be caused from a dirty fuel filter. Its a 93 Thor Pinnicle, chevy chassis, 454 with a throttle body. Where is the fuel filter located?? Any other ideas what could cause this? Any help is appreciated. Thank you.
Glenn

Something similar happened to me on my Dodge Dakota and again on a Ford van.  Both cases were caused by a dirty throttle body.  The first time it happened (Dodge) I had the dealer fix it.  The second time, I did it myself by just spraying carb cleaner in the throttle body a few times.  Don't know if your problem is related, but it's pretty easy (and cheap) to try.

Fuel filter should be on the frame. Also check the TPS sensor.
J

Ill try both. Thanks

Look at the plug wires, especially the Spark Plug boot on # 8 cylinder. The 454 engine has a tendacy to get an exhaust leak which is noot bad enough to hear yet but burns the plug boot on #8 cylinder. NAPA sells plug wires with ceramic boots to alleviate the problem but they are expensive, Eventually you willo have to deal with the manifold leak if that is the case.

THanks GW, I will look at that too. I changed the fuel filter, and you should have seen the crud that came out of the old one from the in-flow side. Brown crud. I also cleaned out the throttle body with some carb cleaner,,,,,,nice and shiny now. Ill take it for a spin later to see if those two things had any effect.

When you cleaned out the Throttle Body, I hope you opened the Throttle Valve and cleaned around the Throttle Plate as well. Over time the crud build up around the plate causes rough idle and hesitation. BTW, I had a 93 Thor Pinnacle which did have the exhaust leak, also seen the same problem in customers trucks. The fuel filter is way undersized also so they do clog up fast.

I did not GW, but i will. I just got back from a drive and it still has the backfire, and loss of power, even hesitation. BUT, i noticed it only does it after it heats up to operating temp.....about 4 or 5 miles. If it was something to do with the sparkplug boot, wouldnt it do it all the time?? I will let it cool off and open up the throttle and clean the plate.

Mine started out only under heavy load such as the mountains. It progressively got worse. When cold you tend to run richer, after warming up the computer will lean out the mixture. GM systems just about always fail rich. If you have a O2 sensor or map sensor failure you should see a check engine light. When you go back in look at the plug boots especially on #8, but also check #1  Another thing to check is the rotor inside the distributer as GM has had problems with them also.

Thanks again GW. Which side is even and which is odd? Been a while since i had shop class if you know what i mean. My motor is one made in towanda or what ever that name is. Looks like i have to come up from underneath to check the plug wires.

Left side sitting in the drivers seat is odd so the front left is 1. Don't worry, I don't work on them or teach them any more so I have to stop and think. While you are at it go ahead and check all the end cylinders, ie 1,2,7,8. Hope you find it. I am oon a very slow connection here in VA so it takes awhile. We are between Charlottesburg and Fredricksburg near Louisa. It was raining but appears the sun may be coming out. If it is exhaust leaks you will probably see swollen plug boots or they will crumble when you touch them. Another possibility is a worn cam, bent push rod. I am a little slow with my thinking anymore so bear with me.

Check around for a vacuum leak especially the throttle body. The fuel filter should be in the fuel lone between the tank and the engine, also the fuel pickup in the tank has a screen. The throttle body can be cleaned but do not use carb cleaner as it removes the protective coating. Be sure to use throttle body cleaner.

How many miles?  That is a very common problem that can be caused my worn cam timing chain.

54K donn. I just pulled my rotor and hopefully attached the pic. Its black around the center. What do you think? All the spark plug boots look good, in fact , each one had a metal shield around it. Looked for vaccume leaks, didnt find any.
Hmmm, said my file i was trying to upload was too big. Anyway, the rotor is black around the center post contact point, like its shorting out or burnt. Ill replace it.

Take the rotor off and look at the underside....If you see the telltale black spot with white halo, replace it.

Replaced the rotor and the cap. checked all the plug boots, check for vaccume leaks. Nothing. Still backfire thru the throttle body AFTER the engine is warmed up to operating temp. Also still noticable loss of power. So far i only have about 40 buck into it counting the new fuel filter. Not sure where to go now.

OHM the TPS.
J

Back to some basics... backfiring through the throttlebody is caused by firing of combustible material in the cylinder when there is a path to the intake manifold open, allowing gases to enter the manifold.
1. Possible causes.
    Electrical system problem:
        as mentioned, bad wires and arcing between them or some other component allowing cylinder to fire when the intake valve is open.
        timing changes caused by worn cam gear or timing chain moving one or more ignition points to cause detonation with a valve open.
    Intake valve sticking open (since it happens when engine is hot, may be the culprit - bent valve or weak return spring changing dimensions enough to bind or not allow closure of the intake valve.
2. How to identify:
    Electrical system problem:
          bad wires or arching
              Operate engine in the dark - look for arcs between wires (it can be pretty but causes problems. Wiring can look good and still fail. 5 years/50K miles is a good point for your engine to consider replacing them as a maintenance item.
              You can inspect the boots for "carbon trails", thin black lines indicating rubber that has been converted to a path for high voltage electrical voltage to travel.
          Timing changes should kick a fault to your engine control computer, but only way to ensure timing is correct is to hook a diagnostic computer up to engine (old folks would use a timing light). Of course included here is possible bad ECU. The computer should be instructing engine when to fire.
          The only real way to identify sticking valves is to perform a pressure check on each cylinder or even better to perform a cylinder blow-down test.
3. Repair depends on what you find.
4. Based on also having a loss of power when engine warms up, I would expect timing or sticking valve.
IMO I don't believe lean fuel should cause a backfire through the throttlebody . A vacuum leak, if the distributor is vacuum controlled advance, could cause problems including backfiring.
Good Luck

As Bob said, at this point you really need to do some mechanical tests. A compression test or a cylinder leak down tests will tell you a lot about the mechanical condition of the engine. We have pretty well adressed all the simple things to check for so now it is time to get the hands dirty.  Good Luck!

THanks all. Looks like ill be taking it in. I have done all i can do myself. I appreciate the help.
Glenn