Need Help on HP Problem

[mrviper99]

I have a 1996 GTS, 10,000 miles and the following mods.
Larger throttle bodies
Smooth tubes
K&N Filters
3.45 Rear End
Hennessey Headers & Exhaust 3"
Hi flow cats
1.7 roller rockers (TNT)
Heads flowed and larger valves (TNT)
Street-Performance cam (TNT)
Aluminum flywheel
My problem is 2 weeks ago I felt a miss when punching it in mid rpm range. So I changed plugs and wires. Felt it was needed anyway. Yesterday I attended my clubs dyno day event. I ran with not to bad of numbers. 492 rwhp 512 rwtq. However in the middle of the run(4200-4500rmps) I have a horsepower loss dip and a torque dip. The horsepower dip is close to 50hp and last about 200 rmps. The torque curve does the identical losses(about 40 ft lbs). After 4500 rpms the curve climbs and stays smooth. The dip is right around 400 hp and the torque is around 500ft lbs. We had a air fuel monitor on the dyno, however it was just inserted in the exhaust pipe. On the left bank we stayed in the 12 to 13 range the right side though dipped down into the 11 to 12 range, once early in the run at 3000 on again at 4500. So my question is what is causing this is it a spark problem, a/f problem, air turbulance in the the intake or what? The dyno tech said he could feel the miss and others in the group said they could hear the miss. When I get to work on Monday I'll see if I can scan and post my dyno run for a better look for every one. Thanks for any help you can give. Jeff

 

[Marv S]

There is a TSB on the '96 GTS for intermittant power loss between 4-5k rpm. The fix is for a dealer to flash upgrade the ecu and put a sticker on the ecu cover (or near-by) noting the authorized flash has been done.

TSB: 18-15-97 or 181597
Make: DODGE Model: VIPER Year: 1996
Service Bulletin Number: 181597 Bulletin Sequence Number:

Summary Description:INTERMITTENT MISFIRE AND/OR MIL ILLUMINATION WITH NO DTC'S. *TW

One or All of the following symtoms may be experienced:
Intermittent hesitation between 4 and 5 thousand RPM.
Intermittent malfunction indicator light (MIL) with no diagnostic trouble codes (DTC's).

 

[mrviper99]

Hi Marv.
Thanks for the reply, But I did have the computer flashed back when the TSB came out. I do have a service engine light on and the code is 52 a running rich code. Thanks again.

 

[jcaspar1]

A small dip in torque at this rpm is normal for Vipers but yours sounds excessive. The normal dip can be tuned with a VEC 1. Post your dyno sheet if you can.

 

[racetech]

These kinds of problems can be tough to diagonse and really frustrating. You should exhaust all possibilites in the ignition & fuel systems first (don't forget to check the injector coils with an ohmeter--they can develop low resistance)...but I have seen valve train harmonic problems cause symptoms just like what you're describing.

If you broke an inner valve spring, you'll still have a spring, but weakened and less-able to control harmonics. The check is pretty easy--remove the valve covers and make a visual inspection of the springs (particularly the inners), then check the tension of each. You'll need a valve spring tension tool that grips the rocker arm.

Bump the engine over so that the lifter is on the cam base circle. This happens on the exhaust valve when the intake valve is at least half closed. The intake is on the base cirle when the exhaust valve begins to open. Once you have one on the base circle, pull on the tool enough to open the valve and read the tension. Replace any springs that have lower tension than the others.

A much more uncommon problem can be the choice of springs. Back in the 70's, RM had been running their Corvette with world-beating results. Then they freshened the engine and installed valve springs that were supposed to be the best thing since candy. The engine would nearly stall in the intermediate rpm range--there was so much intake reversion that a cloud of air/fuel would actually stand off over the carburetors. When they put the old valve springs back on, the problem disappeared. This is a longshot for sure, and mid-range rpms for the RM people was more like 7000, not 4200.

Hope this helps...

 

[mrviper99]

Thanks racetech for the reply; I don't think its the valve springs but will inspect tomorrow. Have a feeling it's something else but not sure yet. Will try to scan dyno run tomorrow. Thanks

 

[mrviper99]

 

[mrviper99]

Yea I got the Images to work. Finally

I'am going to see my viper tech tomorrow to see if the computer lost its upgrade. Thanks for any more suggestion of what the problem could be.