Hey guys, I have a 91 L98 with a six speed. My mods are as follows, Ed Wright stage 2 chip, 160 thermostat, K&N filter and open lid, ported intake, 24 pound injectors, adj fuel pres reg, airfoil and double platnum plugs. I just added the airfoil today, and noticed some strange things (I dont think from a result of the airfoil) The car runs (idles) fine, and runs like a bat outa hell, sometimes, but more often than not, I am noticing a flat spot at 3200 rpm's and the car seems sometimes slugish after that, and somtimes it runs really strong. It acts EXACTLY like the A/C comes on. The A/C is not on though. I just noticed it because of the flat spot. I wonder how long its been doing it, throught the rpm range, and I just havent noticed. I did notice that sometimes it feels stronger. The car doesnt use a drop of oil, and I dont know where to look. Should I replace the distributor? Would that explain why it does it sometimes? Seems the more I drove it, the weaker it got. It only gets week to a point, and still revs high and doesnt spit or sputter, but just doesnt have as much power as it did...Again sometimes it comes and goes, and sometimes, it is weak throught the range...Any ideas were to look? My fuel pressure is set at 47 psi.

 

It sounds like a knock sensor to me.Check your timing.
Also if you got loud exhaust sometimes the knock sensor will activate.
I read something about getting one from an LT4 or something.I hope someone eles can varefy that. :chevy

 

I have this same problem. I have an airfoil too. Don't know if this has anything to do with it. Hope somebody will chime in with some possible answers.

 

I' say fuel or timing.

If fuel flow drops off it'll show up as a flat spot. The best was to check it is to install a fuel pressure gauge with a long hose and tape it to the outside of the windshield. Then do a blast whil watching the gauge. If pressure drops, it could something as simple as a partially clogged fuel filter, or a weak pump.

If the knock sensor detects what it thinks is detonation, it'll pull out timing, which will cause the engine to feel flat.

Just my thoughts.

Jake

 

sounds like a bad fuel filter or its picking up knock

 

That is my problem (FOUR YEARS), looks similar to your. You notice that loud exhaust when car run bad and quiet when run good?
that is from my topic:
Since I have my vette (4 years) there is same problem: sometimes car run normal and it is for about 98% times I drove. But there is that great 2% when car runs MUCH better like rocket. When that happen I can feel that engine running much smoother, feel much stronger and much quiet. Whet it run better I can see that my instant fuel economy is better about 20%.
That 2% great running happen when it want, there is no correlation with weather, with temperature, no matter car run long or just started, simple that better happen suddenly and last sometimes 1 minute, sometimes one hour and gone as come: suddenly.
For 4 years I was trying pinup that problem (I want that 2% all the time!!!)
I changed plugs (3 times, ignition cables, rotors, caps, ECM and another)
Still nothing.

 

sounds like I am not the only one with this problem. I replaced the dist and button today, still doing it, and YES I noticed today the exhaust sounded different, not alot but enough to notice. VERY strange. I should add I have a brand new fuel filter, and the car is set to 47 psi at idle. This is the first I have noticed of it doing it, if anyone has any other ideas let me know. The timing is still stock, but the chip may have bumped it. I did not really notice it before the airfoil was installed, but that could be coincidence. Any one else???? :bb

 

I had the same situation when my pre cats were going bad! I eliminated the pre cats and installed a high flow main cat and the car ran better than ever :cheers:

 

I have no precats and cats at all. And still have that problem
I think it is injector or valve?
Remember: that happen sudenly: in one moment car run as always, in another moment run like rocket.....
Sorry for my English
Jerry

 

Check the 'air accordion hose' that goes to the throttle body. Betcha' you tore it during installation of the air foil? :blueangel:

 

i also experience a similar problem

every so often the poformance is greatly decreased.
it feels and sounds as if someone stuck a sock in my intake

of course it never happens when i'm data-logging, but one guess is it's
cutting out timing. it only happens at WOT so i'm also wondering if
my o2 sensor is not getting hot enough. time for a heated one.

a while back after first purchasing the car, it ran mostly bad, with every
once in a while running real strong. i don't know what the problem was,
but i ended up changing the cap, rotor, ignition module (accel), coil (summit
performance), plugs, wires, fuel filter, o2 sensor, and checked exhaust back
pressure.

I broke down and took it to a friend at the Pontiac dealer where they cleaned
the throtle body and adjusted the timing. Still ran bad. check fuel pressure
while driving and even the resistance of all the injectors.

i was lost, so i decided to check the base timing again. now i'm pretty sure i
checked it before i started replacing those parts, but when i checked it after
the dealer did, it was 6* AfterTDC. i set it correctly and the car ran great.

oh, be carful of the connectors to the distributor, they break easy after
12 years....

 

At WOT, the system ignores the 02 sensor and runs off pre-programmed maps in the PROM during PE mode, Power Enrichment.

It could be that the knock sensor is picking up what it interprets as detonation and the ECM is retarding the ignition timing. The engine will feel flat until the timing is put back in by the ECM. Remember, the ECM pulls ignition timing out a lot faster than it puts it back in.

You'll need a way to log this real time at WOT to see if this is really happening. I use Diacom to log my WOT blasts and later play it back to see what's what.

I've never used a ScanTools, so I don' know if it has this capabilty.

Hope this helps.

Jake

 

my car has similar problem. you mentioned that the ducting between the MAF and TB could be torn. well, this is where i installed my relocated MAT sensor. i just kinda tore a hole in it and shoved the new sensor into the plastic ducting. would this cause a big enough air leak to add "false air" and cause this kind of problem? it looks pretty sealed but i didnt add any sealant or anything to make sure it was airtight.

 

i did a dyno pull that CLEARLY showed a big flat spot around 3500... no TB butterfly cuz i have holley 58mm... and no MAF either cuz i have 90 with speed density... i know it's not the injectors, they're new... might be anti-knock??cuz of loud exhaust? never heard that one before... except that it goes away as quickly as it comes... i would post my dyno sheet but don't know how. lets just say... big drop around 3500 or so and then climbs right back up like nothing happened. only other symptom i know of is that the TPS wasn't adjusted properly at the time (WOT was 3.7V) and that when i've done a fuel pressure test, the pressure jumps up to 46 or whereever i have it set and then as soon as the pump stops running, it quickly falls to 20 something and stays there. my buddy's car by comparison jumps to 44 and holds forever... car starts fine and runs well otherwise... new fuel pump in jan. any ideas? i think i'll post this as a new string and see what we get.

 

There are a few different things that can cause fuel pressure to drop very quickly.

Either at the pump or at the intake.

You can use a pair of pliars to squeeze the rubber fuel lines lower passenger side front of engine.

If you squeeze the return line and pressure holds, but when you release the pliars the pressure drops immediately, the the problem is in the tank, usually one of the hoses on the pump. If the pressure does not hold with the return line squeezed shut, then it's got to be at the intake end.

Either a leaking injector, fuel pressure regulator diaphragm or hard line connection.

Speed density systems are not nearly as adaptable to engine changes as are MAF systems and I bet the engine is seeing a lean spot that it's not able to compensate for. I put my money on the big 58 mm TB as the culprit.

I tried a 58 on my 415 last week in place of the stock 48 mm and had all sorts of open loops flat spots. Until the engine went closed loop (MAF system) and the ECM was able to compensate for the increased air flow, the engine would seem to completely die. I'd step on the gas pedal and nothing, then the car would accelerate only to go completely dead again. This kept up the entire time the system was in open loop, but once it went closed loop and began getting 02 feedback, the car performed normally.

So, since in open loop, the engine operates on pre-programmed maps for ignition timing and fuel the PROM was just too lean to support combustion.

I put the stock MAF back on and everything returned to normal.

Hope this helps.

Jake

Jake


[Modified by JAKE, 10:16 AM 4/29/2002]