l05 powerloss/light idle surge

My '92 Rmw has been having issues for a while. the main one is a severe/intermittent loss of power. usually between 1500 and 2000 rpm is when the power loss is exceptionally noticeable. i dont want it to sound like a beat the crap out of my car, but it wont even chirp the tires from a dead stop. if you apply about a 1/4 throttle it does okay putting you back in your seat but as soon as it touches 1500 pm it falls flat on it's face. after 2000 rpm it sorts itself out and does fine.
also, the idle in nuetral/park surges a bit by about 200-300rpm at most. it idles smooth in gear.

heres some info:
its has had a recent tune up (plugs, wires, cap/rotor)
it also has a new ignition coil in it.
the EGR has been disconnected long ago and capped off
new coolant temperature sensor
new O2 sensor
no codes have been thrown
it passed an emissions test recently

maybe the timings a bit off??

any insight guys?

make sure you have good fuel pressure clean filter etc,.could be a vacuum leak,.
tweak the fuel pressure spring on the injector pod,.

thanks nick,

forgot to mention it has a new fuel filter. any tips for tweaking the FP spring on the injector pod? i have no experience there...

The issue is probably the pump hose in the tank. If it has split, it can cause the idle surge, as well as acceleration issues. when the pump twists the hose, it opens the split, and the fuel pressure drops. I had a similar issue when I got my 92 OCC. It did not throw a code. Tweaking the spring will not help you if the issue is the hose. The easiest way to measure the fuel pressure is to install the fuel lines from a 91 or 92 FW with a Chevy engine...they have a Schrader valve port on them.

If you are interested in increasing the fuel pressure, there are some split rings for key chains that will fit over the adjuster in the regulator, and large enough that the spring will sit on top of it.. I am not sure what the diameter is, but I bought one a couple of days ago.

ICM might be a good part to throw at it since you've done the other ignition stuff.

I think power loss was a factory option.

option PL1

Since it's surging at idle as well, I'd agree with a vacuum issue.  Part of the surge tends to come from the ECM trying to compensate the rpm via air flow.

(I'm trying to remember if the 92's had an Idle Air Control (IAC) of some sort on the TBI cars. I know it was part of the LT1 setup, under the throttle body.  It's been years since I played with TBI.)

Anyway, if so, it is usually how the ecm tries to level out idle by running the IAC  shaft in and out. Once again, the cause could be a vacuum issue that the IAC is fighting to control via changing what air is allowed in.

I can see Fred's point about the fuel lines, What does it do above 2000 rpm? If there's not enough fuel I'd suspect the farther up the rpm goes the more starved the engine becomes. It should be falling on it's face at 4000 rpm it that's the case.


Dave

the fuel rate will over come a vacuum leak at higher throttle openings, because as we all know the vacuum drops right off as the throttle is opened,.
vacuum leaks are most noticeable at part throttle,.

lakeffect wrote:

(I'm trying to remember if the 92's had an Idle Air Control (IAC) of some sort on the TBI cars.  I know it was part of the LT1 setup, under the throttle body.  It's been years since I played with TBI.)

They do. Same IAC valve as my Grand National by the way.

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I need the pigtail for the IAC on mine. It's all kinds of fubar'd.

jayoldschool wrote:

ICM might be a good part to throw at it since you've done the other ignition stuff.

Agreed. I had to replace them each time I did a tuneup on our TBI's.

phantom 309 wrote:

the fuel rate will over come a vacuum leak at higher throttle openings, because as we all know the vacuum drops right off as the throttle is opened,.vacuum leaks are most noticeable at part throttle,.

Thanks for the additional clarity Nick.

Dave

Gastt wrote:

I think power loss was a factory option.

came standard  

Well I haven't had time to work on the problem. I've really been concentrating on what exactly happens when the gas is applied though. Like I said the problem is intermittent but when it happens you can hear the engine bog usually around 1500 - 2000 rpm. When gas is applied in this range the rpm doesn't change either. If you floor it durring the bogging or at least give it enough gas to kick down out of overdrive, it will accelerate normally.

Possibly related, I had to use the defrost this week and noticed my HVAC is screwed. Hardly any air moving when the fan is on full. When the HVAC is off I can still feel heat coming from the floor vent. Correct me if I'm wrong, but I think vacuum for the HVAC is taken off the back of the TB. I checked the vacuum connection there and it was fine...

I'm thinking my possible problem(s) are:
TPS
IAC
big azz vacuum leak
Fuel pump line in tank is split.