Would a flat cam cause intake backfire?

I had my 87 into a muffler shop on Saturday and after over 500 bucks of new exhaust system including cat and new shocks I still have intake backfire. The mechanic thought I might have a sticking valve or a flat cam.

I have put a lot of money into this ol girl over the last year and have replaced all of the emission control system with the exception of the air pump and purge canister.

Ok here are some clues that might spark some idea's:
I was told at one point by a different mechanic that my fuel presure was low:
I had 36PSI. Book says 40 to 47. I replaced the injector O rings and a general cleanup. I now have 40PSI.

I have replaced all the gaskets on the upper end with the exception of the Heads and all of the vacuum lines and I seem to have a vacuum leak I can't find.
I have 15Hg @ 1000RPM but my idle will drop to 600RPM then return to a 1000. The vacuum seems to follow from 16 to as low as 10Hg and back. If I WOT the vacuum drops to zero then gets as high as 24Hg then will drop to 14 to 16Hg.

I notice that the intake backfire happens mostly under a heaver load and when the I punch the gas. At cruise it runs fine. The car does feel a little under powered.

Any Idea's?? GMTech??

I have had this to more than one mechanic over the last year and had at least 5 diagnotics with now solutions.

Thanks for any help you can give me.

 

The fuel pressure might be an indication of a plugged filter or a failing fuel pump. I would get that fixed first then go from there.
The back fire through the intake could be a sign of that vac leak you cant find or a lean condition caused by the improper fuel pressure, but since thats fixed most likely its a vac problem. A flat cam would do that also but its unlikely the can is flat unless some one did a "HiGh PeRfOrMeNcE" valve adjustment. But then it would backfire constantly not just under WOT. I would fix the Vac leak problem 1st and then work from there.
SSC

[This message has been edited by SUPER-SPORT-CHEVY (edited May 21, 2001).]

 

Have you cleaned the Idle Air Circuit unit?
Your vacuum is quite acceptable.

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

 

Altho that other stuff should be checked first, how's the timing?

If you have a dial gauge indicator & stand, you could pull the valve covers (I know, what a pain!) to see how your cam lobes are doing. From the stuff I dug up about stock camshafts, the 85-up 2.8 cam had the following specs for "gross lift":

Intake gross lift = 0.393
Exhaust gross lift = 0.410

Is the brake booster leaking? Plug up the hose going to it, then check the car's vacuum again...

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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

 

Thanks Tom, Super and Ked.

Ok here is a sensor list that have been replaced over the last 8 months:

MAF, IAT, IAC, TPS. The Throttle Minium Air Position and TPS Voltage setup was done using Vader's guide he posted in March of this year.

I have already replaced the fuel pump twice. The fuel relay and fuel pressure regulator once along with several fuel filters. #1 injector was replaced because it was leaking which was why they found the contents of my cat in my muffler. The fuel rail has been off and on more than I'd like to say and I would like to be looking at 45PSI but I think 40 is acceptable.

I have had the EGR valve replaced along with the EGR control selenoid. The PVC valve as been replaced as well. The only two items on the emission system I have not replaced are the Purge Canister and the Air Pump.

The distributor has been replaced within the last 6 months along with cap, wires and new coil.

Ok so let me put this another way:
At startup in open circuit the car will not fast idle. If I hold my foot on it at 2000RPM the idle will increase to 25-2800 on it's own and then decrease. After a minute or so the car will idle on it's own @1000RPM but every 30 seconds or so drops to 650RPM for 10 or 15 seconds then returns to 1000RPM. After I hit closed circuit the car idles steady at a light for 30 to 45 seconds then drops to 650 and returns to 1000. If I have to take off when it's idling at 650 I get intake backfire everytime. If I'm idling at 1000 I don't.

The only thing I haven't done is to pull the valve covers. Unless someone has another suggestion of what this might be I guess I'll go there next.

Oh I forgot... TomP the timing has been set to 10BTDC.

[This message has been edited by billste (edited May 21, 2001).]