Backfires at all RPMs

[86GN-862+2]

I did a cam swap to my car some time ago. Cam is 208/208. Mechanical hydraulic. Had a problem with the pushrods lenght which has been addressed. The engines fires right up and it runs normal temperaturewise. Took it for a spin but it backfires at all RPMs ! Didn't get to register much on the scanmaster cause I didn't press the gas pedal enough for it to record how it does. BL read 138 at idle while INT is close to normal at around 124. When I took the car out, I had to shift manually to keep the engine from backfiring if I pressed the pedal just enough to get the car moving. I wonder if I may have stabbed the cam wrong being how the crank gear had three keyways. I just did TDC 0• on the install thinking it should be fine. The cam sensor was stabbed accordingly as well. As I said, it fires right up and idles fine except for the excessive misfires. Could I have done the cam at 180• from specs?
Any ideas? I want enjoy my car once again! Thanks in advance.

 

[Chuck Leeper]

Cam in 180* out= no run.
Cam sensor in 180* out = runs, but not well.
Plug wires on the wrong towers vs cyls=constant misfires.
"Mechanical hydraulic"?? One or the other. Which is it?
Excessive preload on lifters = misfires.
Backfires... Thru the intake or exhaust???
Everything plugged in as it should be? [ground wires, etc.]
Run a compression check after you have eliminated all the quik/easy stuff.

 

[86GN-862+2]

Backfires at the intake, hydraulic, all plug wires double checked. Ground wires nice and right. Had a problem with the heads being out of whack causing excessive preload on the lifters. Addressed by adding shims(.020 all around!) Misfires up top. Maybe still too much preload? Didn't Tripple check that for ALL the valves. Previous owner had the engine rebuilt by some idiot mechanic who didn't know much about TBs. I have replaced a number of engine parts which are failing gradually. The SM does not register any change in timing to stop the misfires. It misfires even at idle. Thank you for the responses!

 

[Chuck Leeper]

??

Backfires at the intake, hydraulic, all plug wires double checked. Ground wires nice and right. Had a problem with the heads being out of whack causing excessive preload on the lifters. Addressed by adding shims(.020 all around!) Misfires up top. Maybe still too much preload? Didn't Tripple check that for ALL the valves. Previous owner had the engine rebuilt by some idiot mechanic who didn't know much about TBs. I have replaced a number of engine parts which are failing gradually. The SM does not register any change in timing to stop the misfires. It misfires even at idle. Thank you for the responses!

Intake valves are not closing...
Heads "being out of whack"?? As in had the decks milled waay too much? That, or the deck being milled too far, p rods too long, would be about the only reasons I'd see for needing to use shaft shims...Put the correct length p rods in, and remove the shims.
The engine misfiring, does not mean the ecm will pull timing. No knock sensor activity, no timing pulled.
Get the preload right, then check the compression. Make sure there's no bent valves.

 

[TurboBuRick]

Valves not opening enough? Not enough preload. or worse a worn cam lobe.

 

[86GN-862+2]

Valves not opening enough? Not enough preload. or worse a worn cam lobe.

This was the reason for the cam swap. By the way the engine behaves, seems as if the heads were milled quite a bit while leaving all else unchanged. BL jumped to 142 at idle with pushrod change. Pushrods I bought with the cam were 8.675 .080 but swtiched back to the ones that came in the engine. Spark plug wires set to 1-4, 5-2, 6-3. All parameter on the SM check out except for the BL which shows a lean condition. Lean = backfires too right? Gonna recheck preload once more and fuel pressure. No knocking and idles good without misfires. I put it in gear while parked and pushed on the gas pedal some... no misfires. It only does it when I take it out on the road. I replaced the MAF sensor not too long ago along with plugs plus wires. IAC set at .44 idles 700-750 rpm. Temp on the SM is under 170 degrees. :frown: So close and yet so far! MAybe I need some adjustable rockers as well?? To make finer adjustments to preload... right now I am using shims to get there.

 

[bison]

This was the reason for the cam swap. By the way the engine behaves, seems as if the heads were milled quite a bit while leaving all else unchanged. BL jumped to 142 at idle with pushrod change. Pushrods I bought with the cam were 8.675 .080 but swtiched back to the ones that came in the engine. Spark plug wires set to 1-4, 5-2, 6-3. All parameter on the SM check out except for the BL which shows a lean condition. Lean = backfires too right? Gonna recheck preload once more and fuel pressure. No knocking and idles good without misfires. I put it in gear while parked and pushed on the gas pedal some... no misfires. It only does it when I take it out on the road. I replaced the MAF sensor not too long ago along with plugs plus wires. IAC set at .44 idles 700-750 rpm. Temp on the SM is under 170 degrees. :frown: So close and yet so far! MAybe I need some adjustable rockers as well?? To make finer adjustments to preload... right now I am using shims to get there.

Shims? What are you shimming? The way to adjust preload is ordering the correct length pushrods. What length are you getting when checking the length with an adjustable pushrod and zero preload/zero lash? You should not need to shim anything and are likely asking for problems if you do since you will be changing the geometry of the valvetrain.

 

[bison]

Not enough preload.

It wouldnt be this since you can run a hyd lifter at zero preload and it will not hurt performance. Maybe noise at low rpm but not a loss of power. Too much preload is most likely the problem if its actually the valvetrain. Sounds like a lean out though.

 

[bison]

Previous owner had the engine rebuilt by some idiot mechanic who didn't know much about TBs. I have replaced a number of engine parts which are failing gradually.

Fwiw the proper way to set up a valvetrain is very similar for all ohv "v" engines. Pushrod length is the most critical on a hyd application. The preload is not adjusted by shimming the rocker shafts up. What other parts have failed that you have replaced or are failing?

 

[bmackinnon]

Had a problem with the pushrods lenght which has been addressed.

Sounds like this needs further 'addressing'.... agree completely with 'old man' Leeper tongue above who said 'valves not closing'.... causing backfire all the time... valves could now be bent...

 

[TurboBuRick]

It wouldnt be this since you can run a hyd lifter at zero preload and it will not hurt performance. Maybe noise at low rpm but not a loss of power. Too much preload is most likely the problem if its actually the valvetrain. Sounds like a lean out though.

Yes Bison but we don't know how much the valves are getting open.

Backfiring at all rpm's sounds to me like the popping we here when the cam has a worn lobe. Hence valves not opening enough.

Can't think of any reason open valves would cause back firing. Loss of power yes but back firing?

I was thinking pushrods are too short, together with the washers under the rocker shafts are limiting how much the valves are opening. The car reves fine in the drive with no load but as soon any load come on, it's backfiring.

86GN-862+2, does it sound more like popping from the intake? Does it get worse when you try and push it? Does the rhythm follow the rpm's?

 

[jdpolzin]

Out of curiosity how much pre-load do you have with the pushrods you are using in the motor now?

Also, have you check the coil pack and the ignition module? I would think it would be fine if it idles fine but we had a couple issues with the coil pack and ignition module miswired between them. This changed the firing order. The car would idle but once under any load, it would die. Just a thought.

 

[86GN-862+2]

I know I am asking for trouble with the shims but I was in a bind for a driver:redface: I am ordering the correct length pushrods this weekend. Car runs great! Better than when I first got it. Most I've ran it is around 8-9lb of boost and so far I like how it runs. It feels strong chirping into second gear. We had our daily driver totaled by the insurance company and had to get my GN running asap until we buy another car. I am driving it like an old lady:biggrin: Don't want to have it break down again!!
I don't know how much preload or much else. I just know I got a car to go to work....I'll fix it soon guys I promise I'm elated I got it to run this good while working on it under my carport and reading up on the stuff that needed attention.

 

[bmackinnon]

Can't think of any reason open valves would cause back firing. Loss of power yes but back firing?

Say what??? Of course an intake valve hanging open will cause a backfire/pop... spark plug lites and ignition occurs in combustion chamber and travels thru open intake valve and boom!

 

[TurboBuRick]

Say what??? Of course an intake valve hanging open will cause a backfire/pop... spark plug lites and ignition occurs in combustion chamber and travels thru open intake valve and boom!

I don't know if I buy that. It surley wouldn't run fine up to a certian point then start backfiring. If the valves are not closing the motor will not have compression and would barely run if at all.

I stand my ground. No compression=no backfire/pop.

 

[Chuck Leeper]

My $.02..

How about this proposition:
P rods are too long and the lifter plungers are nearly bottomed out.
At idle, the oil psi is down, the valves manage to work as they should.
Engine at RPM's, the oil psi is up, boost comes up, net spring psi is down, lifters pump up, valves are held open.. backfire occurs.

 

[TurboBuRick]

How about this proposition:
P rods are too long and the lifter plungers are nearly bottomed out.
At idle, the oil psi is down, the valves manage to work as they should.
Engine at RPM's, the oil psi is up, boost comes up, net spring psi is down, lifters pump up, valves are held open.. backfire occurs.

In that case, yes. I will agree.