93 suburban backfire

[cjttmyers]

I have a 93 suburban that has started backfiring during load. It only does it during acceleration or going up hills. It has also started surging at idle and misses frequently at idle and road speed. The Tach is also jumping a lot but the engine speed obviously is not changing as much.

This is a 93 with 203K miles and a 5.7l engine.

I have replaced the distributor, cap, rotor button, wires, plugs, TPS, MAP sensor, EGR valve,Temp sensor and O2 sensor. I am at a loss as what it could be. I have tried using starter fluid to look for vacuum leaks and did not see any engine reaction anywhere.

Any help would be appreciated.

[maxwedge]

This could be a bad camshaft also., pull the rocker covers make sure all the valves are opening

[cjttmyers]

Could it be the timing chain? That might explain the tach jumping around. The cam not rotating smoothly?

[60 corvette]

could be your catalytic converter is starting to plug up

[maxwedge]

Rock the crank back and forth with a wrench, you will feel the resistance of the cam at each end of the movement ,that will tell you how loose the chain is.

[cjttmyers]

OK, changed the timing set. Old chain had about an inch of travel. New set had about 1/4". Unfortunately it did not help the problem. Any ideas?

[cjttmyers]

Checking the fuel pump today. Can anyone tell me what the pressure should be? I took the filter off and put the gauge there and only had about 10 PSI. I don't think that is enough but I don't know what I should have.

Thanks

[ronaldk]

i have 93 caddy with 5.7 chevy eng and i have had loads of fun keeping her running right. i will share what i ran across. first its tb injected and all gaskets were bad, on egr valve it has computer controled vac to egr that opened egr at wrong time which caused eng to jump like dead spot.Then i thought fuel pump was bad car would die hit tank pump worked so i changed pump car still died hit tank pump worked, my gas tank is plastic so the ground is through wire to chassis that was that prob i also changed time chain but it was slapping. also changed lots of vac lines.hard to say what your prob is you could have intermit spark you changed a lot of ign parts. intermit is hard good luck hope one of these helps

[cjttmyers]

Also pulled the catalytic converter today. did not seem blocked at all. looked very clean inside and took a leaf blower and air flowed with no restriction at all. did not even have enough back pressure to blow back around the blower pipe which was smaller than the converter pipe. I could block the exit and it would change sound and blow back.

My Dad has a 95 truck with same engine. I took the two sitting side by side and started swapping parts. Changed distributor, and entire throttle body. No change. Checked his fuel pressure and it was same. I even swapped the CPU but had to use my chip. Same issue.

Is there any chance my chip is bad?

[ronaldk]

wow swap of dist takes even ign mod out of pix. I assumed you pluged vac line to egr so it stays closed not good for nox levels but if that prob eng will run better thru all rpms. also i believe you have a knock senor could cause prob. I know 95 is border line for ecms so i dont know if that swap helped. a way to check if you have intake gasket has leak is spray starting fluid around intake if idle increases theres a leak take care not to start fire.

[cjttmyers]

Actually I did try the start fluid trick. I sprayed the intake manifold and all of the vacuum lines all the way from end to end (desperate).

With the ECM from the 95 it would run but there was no power and the tranny did not shift right. I swaped the chips and it ran like normal. Unfortuately normal right now is not good.

Edit. With the ECM from the 95 it did not spit back though.

[ronaldk]

as i recall back fire was your main complant if when you put 95 ecm in and that changed it it would seem you found your problem. not running right is probabliy because brains are not the same advanceauto lists 4 diff braians for your truck by orig # from $77 to $113

[cjttmyers]

The 95 and the 93 box is the same. The ones from Advance don't include the chip. Effectively that is what I did by using the 95 box with my chip. It did not help. That is why I am wondering if the chip itself could be bad.

[ronaldk]

the numbers have to match on the enclosure