Intermittent Jerking between 12-16psi....

[nestamon99]

Hi guys, I need your input on the problem my 91 talon has developed.

The car sat for 4 years due to transmission problems. Finally 4 months ago the car was resurrected after a transmission rebuild. Unfortunately I have not been able to enjoy the car due to an intermittent jerking found only between 12-15psi at or above 5,000 under WOT.

Engine:
stock internals
Hahn Racecraft 20g Turbo
3" down pipe and exhaust system
190 walbro fuel pump
stock injectors
ACT 2600 clutch

The problem:
WOT Acceleration from a dead stop, the car will not start jerking until it reaches 2nd gear with the rpms at or between 5,000-6,500. The problem is so intermittent that jerking never happens at the same RPM/speed/gear every time. The problem will never happen in 1st gear regardless of engine load. If I gradually accelerate without using WOT, the car will not jerk in 2nd or 3rd gear regardless of RPM.

What I have done to fix the problem:
Fuel filter
crushed blow off valve
replaced spark plug wires and coil pack

In confusion, I have performed a pressure test on the intake side of the turbo system to inspect for boost leaks. I pressurized the system by adapting the pressure tester to the intake side of the turbo and capped off the intake hose at the the throttle body. The air travels through the turbo, intercooler and blow off valve. When the pressure reaches 10+psi, a popping sound can be heard from the valve cover and it sounds like air is escaping through the valves. The pressure drops as well. I don't understand how? The intercooler hose at the throttle body was removed from throttle body and capped off, there are no other hoses adapted to the intake manifold. Is it possible for the turbo to be leaking air internally??? Can any one shine in?

 

[BIGBLAZE79]

Have you tried to change the plugs to NGK and gap to .026? Also try the BLT with the motor 30Deg past TDC. Man I've been fighting bucking issue above 15psi under WOT for a long time. I repleced the plugs, wires, coil pack and finally I replaced the plug wires again with NGK and it pulls hard ass he11 now

 

[DougR]

Ok, Guys I'm going to go way out on the limb here.... . so please no flaming. I'm merely taking a semi educated guess...

---"In confusion, I have performed a pressure test on the intake side of the turbo system to inspect for boost leaks. I pressurized the system by adapting the pressure tester to the intake side of the turbo and capped off the intake hose at the the throttle body. The air travels through the turbo, intercooler and blow off valve. When the pressure reaches 10+psi, a popping sound can be heard from the valve cover and it sounds like air is escaping through the valves. The pressure drops as well. I don't understand how? The intercooler hose at the throttle body was removed from throttle body and capped off, there are no other hoses adapted to the intake manifold. Is it possible for the turbo to be leaking air internally??? "----

Ok. So the Car has been sitting for 4 years.... Given some experience with Engines in that state. And having to revive them, (All, NA engines)

...........I'm going to say, Weak valve spring or springs. Due to being in a Compressed or semi compressed state for so long.
What I "Think" maybe happening for him is at his "set" psi for BLT is that a valve is Popping open, Due to insufficent valve spring presshure. Thus causing the leak he is hearing through the valves..(Which side, we do not know atm).

Now onto the "Jerking" he feels while driving. This also could be symptoms of weak valve springs due to Valves blowing open under boost.... BUT! this Could also be in the realm of Valve Float.
So my Q? Is IF! this is happening. why has he not knocked a valve head off and blown the engine... And or heard atleast some knocking At first, I thought.. Fuel cut, and the 20G... Considering his lack of any sort of Fuel controller, But this maybe simply not listed....
But FC is a Sudden Halt to HP production not a Bucking or jerking as he says...???Right??? PLS correct if wrong as I have not had FC yet..

Anyways, Im way out on the limb here...I know Answering a Q with a Q is not polite, But...What do you guys think about this?

 

[kottyking]

fuel cut?

 

[nestamon99]

I have new spark plugs wires, NGK.

Here's the thing that is got me puzzled. Why in the hell would my pressure drop when doing a boost/pressure leak test? Assuming that the turbo is sealed, there should be no air escaping.

When I performed the pressure leak test, I removed the intercooler tube from the throttle body. I then capped the opening, this eliminates the possibility of any air escaping through the throttle body plate. I pressured the intake side of the turbo system by placing the pressure tester on the inlet side of the turbo. This would mean that the intake side of the turbo system is sealed. The popping noise can be heard on the valve cover......

How is it possible for a seal in a turbo to leak air and not burn any oil??

 

[sophat34]

I had the same problem, i was blaming a vpc, untill i took it out and put in the stock injectors. I tried every thing, even with the dsm link and 950cc injectors it didnt go away. tore the motor out and had the head rebuilt with a diff block...havent had the problem since. I would check the valve springs, there not that tough to change with the head still on the car if you have some patience.

when you disconnected the ic hose from the throttle body did you take off all the vac lines as well?

I would give a thought to valve springs, if you need directions on building a valve spring compressor let me know and i will dig mine out. if you have a welder there not to tough to build.

 

[BIGBLAZE79]

Just going over your post again and I was thinking there is no way the air could be leaking past the valves when the pressure is not even entering the intake manafold (capped off at throttle body) Not possible to go through the BOV vac line. The valve springs are not bad. The air pressure is most likely going past the turbo oil seal and going through the oil return line pressurising the oil pan and then up into the valve cover finaly pushing out the PVC line. If you pull out your dip stick and try it again does the air come out the dip stick?