Guys, I'm at the end of my rope with this car and can't find a soul who can help locally so I thought I'd pick the minds here in AT. I have been chasing these problems for months and there's no telling how much longer my hairline or wallet can hold on!

Runs great at idle but above that the car runs rough, like the valves are not adjusted right (have tried them everywhere between 0 and 1/4 turn preload). Car revs slowly and roughly and if you open the throttle too quickly it pops through the intake. Cam timing is good, compression is 185-190#, under 5% leakdown, has maybe 200hp.

The tune is off but I'm convinced this is a mechanical problem. I'm hesitant to throw money into a tune that would become worthless if I abandon this motor, which is becoming a greater possibility by the minute.

357" sbc, Comp R lifters, pro topline 200cc/64cc heads, 10.4:1, xr288hr cam, PBM 3400 springs at 1.750" Fuel pressure I'm not sure about but it has a walbro 255hp pump and the non adjustable regulator that comes with the Holley stealth ram. Every part in the car has less than 60 miles total.

At this point I'd like to hear whatever suggestions you may have!

 

I had 6 bent exhaust valves att the end of last summer that acted like ingition missfire at times then at times it would be fine. My leak down showed less than 4% and the compression had no alarming readings. But if the valve would have been rotated on the seat and not sealing at the time of testing it would have shown up!.

Sooo
I think you might want too think about ripping the heads off, get the valve springs off and rotate the valves while seated and see what you find. Theres no other way around it.
Also put the springs in a tester and see if they are just worn out. I know I have too do springs every other season... ...if they last that long

 

I don't think bent exhaust valves are the problem in this case. They are brand-new Manleys and when I installed them I lapped them in. This is the second set of valves I've had in these heads. The springs were new with the heads and it's done this since day 1. These heads are probably two miles old and have maybe an hour of run time... MAYBE and have done this since new.

I wonder if PBM valve springs are way off their rated pressures. What is the highest seat pressure that can be run on Comp R lifters?

 

have you checked for more simplier things? ie: cracked spark plug, bad wires, or maybe even bad optispark?

just saying that because sometimes what seems like a major problem turns out to be a simple one, just make sure you have all the little stuff covered.

does it do it all the time? or just when its hot, or cold?

 

Ok then the heads have "new" valves..
the other thing done besides "lapping" in the seats is:
...did you check the guide to valve steam clearance?
Could they be too tight and hanging the valve [s] open some?
Two miles won't show "bluing" of the guides yet if they are too tight.

thats all I have for ya good luck brother

 

nosfed, in my experience any backfireing through the throttle body is usually timing related, either spark or cam, it looks like you're sure about the cam being on though. And dont rule out the possibility of a fuel pressure or delivery problem, an extremely lean motor can pop through the TB and cause a severe loss in power. Get a FP guage on that thing

 

I've had half a dozen sets of plugs in the car, and they're all firing so I doubt it's the wires. Operating temp has no effect on the problem. The oil had been washed down a bit so I changed it yesterday.

I didn't check guide clearance with any measuring equipment, but I did wiggle them without the springs. You have me thinking with this line though, so I went out and looked at the valves that came out of the heads. None of them have any wear on the stems, and all of them appear to be sealing. I know the topline machining is shoddy, and now I wonder if the seats are concentric?

Knowing the fuel pressure would be helpful I know, but I'm not sure how it could be low. The pump, regulator, injectors, and filter are all new and it's running rich (most of the time). Under WOT I have seen it puke huge black clouds out the tailpipe. The fuel rails on my car do not have a schrader valve or any place to attach one so it would be an ordeal to get the gage attached.

My car is not an LT1, so it has a distributor, which is a brand new MSD unit, and it had the exact same problem with the OEM distributor. To set the timing, I made all the cells in the spark table 8* and then adjusted the distributor so that it showed 8* with the advance unplugged. Then I switched the spark table back to the base setup. As for the cam timing, I took the front cover off yesterday on the advice of a board member (hawk?) and verified that it's right.

I really appreciate everybody trying to help. I think I've looked at every obvious possibility but something has to be causing this and I have no more ideas. If I can't get it fixed in the immediate future I am going to part it out. I wrote up the ads today!

 

not positive, but i think you didnt set your timing right for the dfi. i have a lt1,but my calmap came with an extra update for calibration of any motor. mine says to " with the engine running, get into the timing map. then bring the engine to 1600 rpm. then use a timing light and physically adjust the distributor until the timing mark on the damper reads the same as the value in the center box of the ignition map screen and tighten the dist. down." now you should be syncronized.

also if you are blowing black smoke,you are way rich!! if you have the wrong timing, (probably too low) and are rich, you can easily loose a lot of power.most cars like 34 to 38 * at above 3000 rpm and idle about 25 to 31*(total) get your timing right and lean it out some and you will be amazed at the difference. also again, check your vacuum! if it is way low(under 10), your timing is off or your t. body is open too much. does it have a map sensor? if so see what it is at an idle, should be below .7 and go down to around .4 with some light throttle. check your tps sensor, is it at .5 or less at an idle? and getting to 80% or more at WOT? check your ecu config. make sure you are in speed density (sdf) and not alpha -n. check your coolant temp sensor, check the air temp correction map to see if it is altering your tune. if you are sure the motor is healthy, then its all in that silver box. no matter what you have,it cant run with horrible tuning. e- mail me if you want to talk further, jay

 

P.S. you said " it showed 8* with the advance unplugged" how can you unplug the advance? dfi uses a uses an electronic advance distributor, it cant work with a standard HEI dist. with vac. advance. so what did you unplug? im not positive, but i think you need a dist from a 90-92 camaro to work properly. it has to have a EST signal. for dfi.

 

Say what? The DFI harness has a wire that controls advance and that wire must be unhooked to set the timing. Otherwise the timing moves all over the balancer. I followed the directions TO THE "T" in the EMIC manual on how to set the timing but I could go through the steps again.

The distributor is the correct one for the setup. It's an MSD made for TPI and I have also run the stock distributor.

 

The car idles at 12.6" vacuum at 950rpm and the tb is pretty much closed. TPS is correct, as specified in the EMIC manual. I am in sd, cts reads right, air temp correction is 0 at 70 to 120 and is above that at left to 4 at 10 and down to -3 at 180. All sensors read appropriately in the "v" screen.

 

Update: I set the valves at .002 preload and it smoothed out some of the rough accelleration. Made a data log and now it's super lean under load. Checked fuel pressure today and it's 30 at idle and 34 at idle with the vacuum off and closer to 30 under load. I'm going to replace the fuel filter and see what happens but it's new and so is the pump so who knows. Unfortunately the fuel pressure regulator that comes with the stealth ram isn't adjustable.

 

Your fuel pressure is too low. It should be 3 bar (43 psi) 30 is too low for the injectors to flow consistently. Get your fuel pressure up to the standard high impeadence pressure of 40-43psi.

Try turning off closed loop and run the system in open loop, this will keep the computer from going crazy while trying to get to 14.7 AFR.

Check and double check your timing. Make sure that the advance, that the ECM is commanding, is what you are actually getting.

Check the map sensor is not connected to a ported vacuum and that the ecm is set right for the right sensor (ie. 1-bar, 2-bar, etc).

If you really think that this is a mechanical problem hook up a vacuum gauge to your motor while it's running. This makes it really easy to diagnose which part is being troublesome.

 

Thank you for your reply. I was doing research last night and apparently the fuel pressure regulator that comes with the stealth ram is known to be junk. Unfortunately it's an odd design so it looks like the only way to replace it is to weld an AN fitting on the rail and buy an inline regulator. Too bad I've already replaced the (*&%%%* regulator once with a Holley replacement So now I'm trying to decide on an inline regulator if anybody has a suggestion. Nice work, Holley

With that in mind, it appears that the car is running rich down low and lean up high. No surprise, considering the fuel pressure issue and lack of tuning.

I actually think the mechanical problems are under control at this point. After lashing the valves again it revs much smoother and does less popping. My guess is that what I'm hearing now is caused by the lean/rich condition, though it does occassionally backfire throughthe exhaust as well.

Could you explain what you mean by this: ? The map is attached to a dedicated nipple that's screwed into the plenum box. It's set on 1 bar and the timing is 100% correct. I set this system up exactly as described in the EMIC manual.

Thanks again, Matt

 

Ported vacuum is engine vacuum that is available above the throttle plates. It is typically use to advance the ignition timing on vacuum advance distriubtors.

I doubt that the tpi has one but is something to check, just to make sure. Hooking map to the ported vacuum will give you incorret map readings.

I looked over the logs and table that you e-mailed me and they look pretty good. You do have some lean spots in the upper end of the table but they shouldn't be causing the symptons you describe.