Chevy 454 backfire at speed

[leomarcotte]

I have a 1990 36' Land Yacht with a Chevy 454 that I purchased a few months back (70K miles). The engine appears to run fine at first with good acceleration and smooth operation.

After about 30 minutes on the road it develops a backfire while climbing grades at highway speeds. Depressing the accelerator to the floor seems to rectify the problem in most instances but it does occasionally still occur. In addition, when coming to a stop and starting to accelerate again the engine idles and runs very rough. While it eventually gets back up to speed the first 30mph of speed increase is very slow and painful. Eventually, it appears to kick in and start to have a more normal acceleration.

The dealer (Huffines Chevrolet in Lewisville Tx) has replaced the spark plug wires, spark plugs, both fuel filters, and done the 60K service that included tune up and injector service. The problem still persists.

Anybody have any idea what is causing this problem?

I am considering a vacuum leak, fuel lines being weak, fuel pump replacement, coil replacement, or possibly bad new spark plug wires. What priority should I attack these or what others should I add to the list?

Thanks

[fangthorpe]

If you also have a high idle (1500-1800 rpms) on startup and a hot idle in gear over 1000, you might check the base gasket under the throttle body. Spray some carb cleaner at the base gasket and see if the idle goes up more. I have never owned a TBI that didn't have this go bad at some point. Cheap fix. Sounds like a bigger leak than just the gasket, though.

[thenewkid64]

I am experiencing the same type of thing.

Starts fine, runs fine under moderate throttle. Hard acceleration or WOT and I get coughing and sputtering and attempted backfires. Filters are fresh, timing is right, new wires, cap and rotor. Could my carb be a bit loose and the extra demand cause trouble??

[leomarcotte]

Thanks for the input. I will check it out as you suggest.

[wb13798]

i would replace the coil. and the ignition module. lol
al

[59toaster]

Getting any check engine lights?

This could be a few things. EGR comes to mind and GM engines are known for this. EGR should be closed till about 1500 RPM. If it comes on at idle it will idle very poor and possibly stall. When accelerating it should also close.

The way the GM FI does it's check on the early FI it may not trip a light when the EGR is stuck open. Stuck closed it will. Quick test is to unplug the EGR vac line and drive it. You will get a check engine light after about 2-3 minutes of steady speed driving when the ECM senses the lack of EGR function.

If yours is set up like mine (88 FI 454 Suburban) it has a control solonoid for the EGR. It will be located up by the water neck. I found the solonoid was sticking and causing a serge. I put a couple of drops of air tool oil in it and so far so good. Newer vehicle use a EGR with a sensor that can tell how open it is but I think yours predates that.

Fuel pump could also cause this but if it's using a typical GM pump they just die and quit pumping some times they will restart and run for a while. Now low fuel pressure could cause these symptoms and cause the engine to lean out. At that point when it started detonating the Knock sensor would retard the snot out of the timing trying to stop the the detonation.

As was said " Check for Vacuum leaks".

If your computer savy and have a lap top you can do real time monitoring of the FI through the ALDL port.
http://www.winaldl.webhop.net/ is a neat one. Verny nice graffics on this system.

lot of good info here on making the adaptor out you can buy one already made. Plenty of other information available.
http://www.akmcables.com/aldl.htm

This guy has some good information as well. I'm trying to get him to let me do a review on one of his systems to retrofit my 4x4. He has a good software package as well.
http://www.customefis.com/

I'll be running a laptop in my Burb tow rig that will have one of the above systems running. Plan is to realtime the engine with it as well as tied into GPS and DVD to keep the kids entertained while we are traveling.

[dobedo]

I had the same problem with my 454, try changing the PCV valve
cleared mine right up.

[overlander64]

I don't know whether this would still be an issue with 1990s era vehicles, but my '75 Cadillac with the 500 cubic inch V8 suffers the described malady almost every spring when it is taken out of storage. The exhaust heat riser becomes sticky and either doesn't open or fully open and as the car warms, the performance degrades until it either backfires and stalls or overheats - - it is now on the list of "must" repairs on its pre-use check-list for my mechanic.

Good luck in locating your engine's gremlin!

Kevin

[59toaster]

Originally posted by thenewkid64
I am experiencing the same type of thing.

Starts fine, runs fine under moderate throttle. Hard acceleration or WOT and I get coughing and sputtering and attempted backfires. Filters are fresh, timing is right, new wires, cap and rotor. Could my carb be a bit loose and the extra demand cause trouble??

Check and make sure the choke is fully dropping. The choke has a lock out device that if it's not fully open it will not let you get the four barrels. It could also have a bad Acclerator pump.

Both are easy checks. Pop the air cleaner off. Shine a light down the primaries (front and may need to prop open the choke if it's cold) give it full throttle and look for fuel to shoot out of the walls of the bores at 45 degrees to the center line. Now quads are known for a problem where if they sit the fuel bowls will sometimes drain. To make sure that hasn't happen Start the engine and let it run for about 30 seconds before looking.

On the choke what you want to do is get it to full operating temp and verify the choke is all the way open. On the passengerside of the carb body is all the choke linkages. mixed in with that mess is a leittle lever the choke lifts that jams the rear butterflies from opening down at the base.

If the choke is not fully open that may also cause the issue.

[Silver Bullet]

Had the same thing on my 460, and I wasn't pullin' the TT. Drove me nuts 'till I checked the vacume advance in the distributor. It was froze to the shaft with grime, a little WD40 and it cleared right up.

Oscar

[George]

I have seen a worn lobe or two on a Camshaft exhibit these symptoms altho it is less likely than the common problems like ign. systems , a vacuum gauge is a good check to do.
I can't believe the G.M. dealer turned the vehicle out with the problem still not fixed. Did you contact them about the problem not being fixed? and what was there responce?
later, ol'George

[leomarcotte]

Yes,
I have had it back and forth to the dealer for the past two months. I originally took it to them to do the 60K service and change all of the hoses and belts, etc.

It started this when I got it back and now i am tired of dealing with them on it so thought I would do some research before taking it back.

I think the real problem with them getting to it is that it does not do this until after it has been driven at highway speeds for a while. The mechanic probably gets it after it has set for ahile and therefore never gets it into the state to make it occur.

Thanks for all of the suggestions. Armed with these maybe i can get it resolved.

[59toaster]

Originally posted by leomarcotte
Yes,
I have had it back and forth to the dealer for the past two months. I originally took it to them to do the 60K service and change all of the hoses and belts, etc.

It started this when I got it back and now i am tired of dealing with them on it so thought I would do some research before taking it back.

I think the real problem with them getting to it is that it does not do this until after it has been driven at highway speeds for a while. The mechanic probably gets it after it has set for ahile and therefore never gets it into the state to make it occur.

Thanks for all of the suggestions. Armed with these maybe i can get it resolved.

SOunds like heat soak. Could be EGR still and the heat cause it to bind and not close. Could be in the distributor ( scratch off Vac advane...all electric on what you have). Could be a coil that has problems when hot.

Knock sensor that goes closed when hot.

Problems like that are hard to find. I would call ahead and talk to the service department. Have the mechanic ready to go when it's hot. He can hop in plug his test gear into the ALDL port and ride along in the passenger seat till it has the problem and see if he can see it fail on the test gear.

[jim8860]

After about 30 minutes on the road it develops a backfire while climbing grades at highway speeds

Is it backfireing though the intake or the exhaust under the load?
Also how is the power under a light load?