VS engine slows down and loses power at 3500-4500rpm

[Fonzie]

Let me first mention this isn't me whining that my car doesn't go fast enough...

95 VS, 3.8 ecotech engine obviously. Only mods are CAI and an LPG duel fuel conversion.
The car was serviced 10 days ago (By a mechanic, not me), it's been doing this since before the service... I left it because i thought a service might fix it. Stupidly, i forgot to mention it to the mechanic though. :bang:

What is happening: From a standstill, or a slow rolling start, when i accelerate it gets to somewhere between 3500rpm and 4500rpm in first gear and starts to lose power. The engine seems to shudder, drops back a few 100 rpm, then picks up again and keeps going. It doesn't seem to happen in second gear, but i can only run it to around 4500 before i'd be speeding, so i can't do much of a test there (Losing my license once is more than enough to learn my lesson). This is when it's running on LPG... It doesn't seem to do it on petrol.

At first i thought maybe coil pack... but it doesn't do it in 2nd gear, or while it's running on petrol. Any ideas? Or should i just get the mechanic to look at it.. Maybe disconnect the battery overnight for a computer reset?

 

[Fonzie]

Bump.
Anyone? It's beginning to annoy me.

 

[camstatic]

Sounds like you're not getting enough gas to the engine to feed it at high load/rpm, get a gas specialist to adjust the flow / inspect the system for the problem.

 

[Fonzie]

Sounds like you're not getting enough gas to the engine to feed it at high load/rpm, get a gas specialist to adjust the flow / inspect the system for the problem.

It 'shudders', then drops back a few rpm, then revs out the way it should... So it can get to the higher rpm, it just... falters in the middle. Also, it doesn't seem to have a problem in second gear OR neutral at the same rpm.

 

[stevieVT]

is it injected gas?

 

[Fonzie]

is it injected gas?

I would assume so... Given that the ecotec's are fuel injected... How can i tell?

 

[stevieVT]

well this just comes from driving gas converted cars, my old man has injected and has experienced no changes loss of power ect, where as my mothers isnt injected and there is a notable loss of power at certain revs during acceleration, this is in comparison to petrol. and i do recall her having the flow being adjusted and a similar problem being resolved.

 

[chrisdoesitbetter]

could be the knock sensor going off try check that....

 

[tadpol88]

could be the knock sensor going off try check that....

that could be right. when the ecu reads a knock in the engine it will switch to limp home mode making it loose power (revs). i think the knock maybe the engine pinging triping the knock sensor etc. id get your gas converter looked at as it might be running too lean. at just a guess i would think that it wont be injected gas system therefor the converter will be only run off vacume. if the converter has rubbish, wax or corotion in it it might just be in need of a clean out.

 

[Fonzie]

could be the knock sensor going off try check that....

What is it, where do i find it, how do i check it, how do i fix it? Sorry, a bit of a mechanical newbie here.