LB9GTA

iTrader: (1)

Here is my story
Have a 400sbc(stock short block with cast dished pistons)
When we put the engine together we didnt check for piston to valve clearance(I have afr 195's)
Once the engine was together I broke a valve against a piston, but the wierd thing was that only the intake valves were hitting the piston. We ground the piston and now there is no issue.
But the engine idles rough and wont go over 3000rpm. The scanner shows no codes but it is showing a lean condition on the o2 sensor(4mv)
I unplugged the o2 sensor and that didnt help. I took a propane bottle and checked for intake leaks and none.
I took the pcv valve off the valve cover and I have positive pressure pushing fumes out not sucking in so it is not a intake leak in the lifter valley.

Any suggestions?
Could it be that the cam was put in wrong?

ASE doc

Did you degree the cam? What does intake vacuum read on a gauge? Perform a compression test. If theres still a question, check cam degree by pulling the drivers valve cover and using a dial indicator. If this checks okay, then look at fuel management.

Two possible fuel management issues that come to mind are fuel pressure, TPS, MAP and coolant temp sensors. It is unusual that a failure that causes this severe a lean condition would not produce a DTC. However, it is not unheard of. Check fuel pressure. I assume you were monitoring O2 voltage on a scanner? If so use the scanner to look at TPS and MAP voltage. While youre at it check out coolant temp as well. Does it all look normal?

LB9GTA

iTrader: (1)

Thanks for the reply!

I havent degreed the cam. I guess I'll have to pull the timing chaine cover off to do that.

LB9GTA

iTrader: (1)

What about a bad harmonic balancer? Wouldnt that make it out by 20-30degrees? It did seem to run better at 25-30 initial timing than the 12degrees it is now


?

ASE doc

The harmonic balancer can slip causing inaccurate timing marks. Slipped harmonic balancer inertia ring seems to be a prevalent issue on our 3rd gens( its the high rpm operation). Thats why I run a Fluid damper. You dont need to pull the timing cover to check cam degree and you can verify timing marks by removing the crank bolt and verifying that the keyway lines up with the timing mark. Then you can degree the cam by installing the degree wheel over the balancer. You may need to remove the water pump for this. Thats why I suggested that you check it by using the balancer timing mark, timing tab and a dial indicator at the tip of the push rod. If your creative you could make this work just to verify cam alignment.

As far as the engine running better with 25-30 dgrees advance, this sounds exactly like a misaligned cam. The dist runs off the cam. Even though the crank may be way off, the dist will still fire the plugs in time with the cam. Again, check compression. This will give you some idea of valve timing. Cold WOT compression o a healthy motor should run about 180. If yours is significantly off. You probably have a misaligned cam. Check intake vacuum with a gauge. It is steady at 18-20in/hg? If not, again you may have a misaligned cam. One tooth off can do this.

Just so you know, I install every cam straight up. I only use the degree wheel to verify alignment is within 1-2 degrees. Some people get really hung up on degreeing a cam. Truth is, advancing a cam 3 degrees increases bottom end torque. Retarding the cam 3 degrees increases top end power. So unless youre builing an engine for some specific purpose, going either way is not of any great advantage and being off by 1-2 degrees makes little difference in an engine's overall power.

LB9GTA

iTrader: (1)

I did some more work last night.

I found TDC by sticking a screwdriver in the plug hole, and it matched the zero timing mark on the balancer.
Then I took off the dist cap, it was pointing between 1 and 8. Then I moved the balancer to 8 degrees BTDC and then moved the dizzy to exactly pointing to #1. Therefore I should have 8degrees of timing.
WELL! It ran like crap, but at least it is not lean anymore! I could barely make it idle and I tried timing with a timing gun, but as soon as I disconnect the EST, the car stalls.

I think I may have to pull the water pump off and use a degree wheel to verify.

LB9GTA

iTrader: (1)

Update

I disconnected the coolant temp sensor and the engine ran better! But it still wont rev past 3500rpm.

Could it be valve float? Possible collapsed lifters?

Ozz1967

iTrader: (4)

May seem simplistic, but your plug wires aren't grounding out anywhere are they? Mine had that problem and it ran like crap till I figured it out.

ASE doc

Plug wires are something to check. Valve float is certainly a possibilty also. Its interesting tough that high CTS voltage, which you would have with the CTS unplugged, would improve engine performance. This tells me you have a basic lean condition. Normally an engine will run like crap and pump black smoke with high CTS voltage.

Your original post spoke of a lean condition. One of the things I asked you to look at was coolant temp. I wasnt very clear about this though. I meant to check coolant temp as reported by the CTS to the ECM. You would use a scanner for this and compare the ECM coolant temp to the dash gauge temp and your own sense to determine if ECM coolant temp seems normal. At this point it looks like there is a basic lean condition since unplugging the CTS improves performance. What's your fuel pressure? Make sure there are no intake air leaks.

You switched from 383 to a 400. Did you stay with the 280xfi cam? Did you make any fuel system changes to go with the changes to the engine?

The idea is to determine if the base fuel system is too lean for this engine combo. My sense is that it is and this will need to be corrected before you can go much further. Assuming youre running factory fuel management, the 91 is speed density and relies heavily on its base fuel map for controlling injector pulsewidth. The MAF system is more forgiving as it responds directly to changes in air flow. With speed density, fuel pressure, injector size, and base fuel map must all be dialed together to a particular engine. You may be able to find some improvement by increasing fuel pressure. You may end up needing to have the ECM recalibrated.

LB9GTA

iTrader: (1)

yup, improper cam install, 3 teeth out

More fixing

Started a new thread for possible more problems
https://www.thirdgen.org/forums/tech...ml#post4624925