Chevy & GMC Vans

I am quickly reaching the end of my rope. The van is backfiring into the intake manifold at speeds of 70-90kph when accelerating slightly or some times holding speed. It will constantly backfire if you don't either get right off the throttle or put it right to the floor.

So far in my attempt to fix the van I have swapped the cap and rotor (the problem started off on rainy days only but is now constant)
I have swapped the TPS sensor and the MAF as well as the O2 sensor. I simply can't afford to keep swapping sensors at nearly $100 bucks a shot. I think it is dumping massive fuel in at half throttle or so and that is whats causing the back fire.

Is it possible that the knock sensor is retarding the timing to the point that it is firing before the valve is closed???

I love my van but am nearly ready to burn it down if i can't fix this.

Any thoughts are welcome.

Kevin

if it is truely dumping excessive fuel, i would suspect the fuel pressure regulator or the injectors in the throttle body before the knock sensor...

I owned a small block in a Chevy Bel Air years ago. Did the same thing, back fire through the intake manifold. Problem was a lobe on the cam for an exhaust valve wore down. The exhaust valve would not open. There was no lift because the lope was round. After ignition it was exhausting the next time the intake valve opened up into the intake manifold. What I did was remove the valve covers and then place them back were they were. Then I started the engine and carefully lifted the valve covers up and looked for a rocker arm that was not moving. Be careful with the cover because there will be oil flying all over the place. Lift it and peek inside don't remove it completely. if this is the case then you need a new cam and new valve lifters. You might want to also replace the timing chain while you have it apart. Hope this helps.

Wow Vinny any more bad news??? I hope it isn't a cam the engine doesn't run rough all the time it just back fires at speed it idles fine and runs strong at wot. If you drop the van into drive instead of overdrive while it is backfiring it goes away as the engine is running more revs.
This has kind of got me stumped. I almost wish it would blow up so i could con the wife into a v8 swap however I think it will take alot of convincing

Does your oil smell of excessive gas?

It could also be weak or broken valve springs. Or bent or broken push rods. Or damaged rocker arms. You need to remove the rocker covers and investigate.

I am just heading out to give it a oil change and swap the injectors since I have some old 45lb injectors left from one of the v8 swaps.

lucky for me my symptoms seem to be the opposite of Vinny I get backfires at steady speed and smooth running at WOT. After the oil change I think I will pull plugs to see how they look before swapping injectors as the van needs to be running in 3 hours. this seems like my life at the moment.

Well I owe a big thanks to Sanj (Reaper) he called about something else and when he heard I was having van trouble he came right over and lent a hand.

I rebuilt the TBI with a rebuild kit from Lordco and it seems much happier. Sanj changed the shift points so that it revs up a little more before shifting as it seemed to be shifting early and bogging the engine building up extra fuel and back firing. With the new fuel pressure regulator baffle/valve in place and the shift point adjusted it is no longer back firing. Of course now it is missfiring a little but I think some new leads and plugs will fix that. I started the engine today with my arm on the dist cap as I was leaning across looking at the fuel spray and got one heck of a jolt thru the arm.

Once again I am amazed at the community we have on this board. Not only did Sanj show up to help just because but he also showed up with a new roof centre console but a new dash from a 94 with a better speedo in it with a aftermarket white face gauge. It looks really cool but the speedo is WWWWWWWWWWAAAAAAAAYYYYYYYYYY out as it needs a new drac module to make it read right. So now I have a new project for next weekend of going to the wreckers to get a drac and installing the new dash again.

To Sanj for the help the poor guy was here till 10:20pm helping play with the new dash adjusting the odometer to the correct mileage and testdriving to check the gauges.