Hey!

So here's the deal, i got my 5.0 TBI formula under full rebuid.
Before the rebuild it was working 100% ok, no issues whatsoever: great (for 5.0) performance, mpg, etc.

After the rebuild i'm having following issue:
If i'm mildly accelerating no matter what gear, when rpms reach ~1600 i get a gap/lag/whatever which feels just like the car stalls or has no fuel.
The rpms start to fall untill at ~1000 it starts to accelerate again.
Now if i floor it it starts to accelerate just fine through 1600 and further.
If i let off the pedal all the way to almost not toching it starts to accelerate too.
Past ~2100 rpms there are no lags.
This seems to happen more if the car is at op. temp or above, but still happens at cold.
I get no check engine light.
I cleaned the fuel system.
Ofcourse the sparkplugs are new.
I've changed almost every electric part/sensor i have:
- fuel pump
- pressure plate (on which the inhectors are)
- all temp sensors
- knock sensor
- throttle position sensor

I've been to many dealers - none know what this is.

Please Help!


Thats how the bird looks now btw:

 

Check the fuel pressure.
Check the ignition timing & make sure it advances when you rev the engine.
Check the ground wires, esp. the ecm grounds. These can be on the back of the heads or t-stat area.
Could a rag or something be in the exhaust system?

 

deffinately check the grounds. and you sure the spark plugs werent dropped and were they gaped properly

 

Timing's fine, checked that not long ago.
All the grounds have been double checked by two different electicians.
The exhaust is brand new, catless, long headers, etc - haven't noticed any difference in flow on either banks.
Spark plugs have been changed right after full fuel system flush (the one with chemicals). No changes whatsoever.

 

ok. but are they known to be good. because they could have been dropped by the parts store or something also.

 

Do you do any other upgrades to the engine besides exhaust?

 

Never seen bad sparkplugs act like this and i've driven cars with very bad ones. They act different, they just make engine work worse.
In my case it's as if the engine shuts down (although it never actually does when this happens)

Good example would be to floor a modern car on ice with traction control which would immediately shut fuel supply!

 

Nope.

 

had a cracked insulator on my LT1 4th gen and it acted similar to the description. it ran fine at higher rpm's but awful down low.

 

I'd bet your running lean, is your fuel pressure adjustable? If so crank it up to 14psi & see if it responds.

 

Which insulator?

 

No it's not adjustible.
And when this happens i can smell the rich condition from the exhaust!

 

I believe it was cylinder #6 spark plug but I checked all of them and that's when I found the cracked insulator

 

What were your simptoms?

 

random sputtering, wouldn't idle correctly. In other words, it acted like a miss in the ignition which is exactly what it was. it wasn't like no plug at all where you can tell you are running on 7

 

my 5.0 was doing the same thing i just unpluged the vacume line from the throtel body to the map sencore i think it was the one that runs to the littel black box by the widsheild and i pluged it with a screw and the car ran fine after a day of driving like that i put it back and the car was fine

 

The thing is mine is great at idle, runs great when i floor it etc.
And again if some wire on one of the plugs was bad, it wouldn't like stall it would just run worse alitttle!

 

Interesting... I don't see why this could help, but since it's easy to to try out...

 

The EGR is opening too soon, it is a very common problem on L03 engines with upgraded exhaust. Just obtain a replacement EGR valve from a L05 truck engine, bolt it on the manifold and it will operate correctly with the lower backpressure. You can test this by removing the electrical connector from the solenoid and driving the car, it will no longer stumble at light throttle acceleration. In extreme cases I have seen the EGR valve open so quickly that the engine will stall as the throttle is opened near idle.

 

My EGR is shut off.
Also i had catless exhaust before, just no long headers - had no problem!

 

Are you still running on a stock chip with the EGR enabled??? If you are that could be causing you to run lean (and rich in places) with the headers. I have seen headers lean out the air/fuel ratio by a whole number. As in went from 12.7:1 to 13.7:1.

Also the stock fuel pressure regulator has a tendency to allow the spring to break and cause the fuel pressure to drop.

BTW--Stock regulator is not difficult to make adjustable either

 

I have some electric thingy that tells the computer EGR is fine!
My fuel pressure regulator showed some leak, so i have just replaced it.
Was kinda hoping it was the problem - unfortunately not!

Once again, leaning out would mean the engine would run worse (or better to some point?) but in my case feels like it stalls! Doesn't look like lean condition to me...

 

Ok but here is the issue with your electronic thingy that I am assuming is a resistor. The computer is programmed so that when the EGR opens the fuel delivery and timing are adjusted to compensate for the dilution of the intake gases with EGR. If the EGR is not there the car will run lean and generally ping like crazy.

 

What would be the easiest way to check this idea?

 

So i tried the following:

The car didn't start very well and threw a check engine light but overall drove great with no lags or any other issues!
Once i plugged the line back into the sensor, the car began starting ok, but the lag came back!

 

Hey, any new ideas?

 

Sorry to jump on your thread but I too have a question about my 89 5.0 tbi. Maybe someone can point me the right direction. So, I drove my car to the nearest carwash after putting insurance on it. Then I decided to try it's performance on the highway. After getting on and accelerating... At about 45mph I felt a lag, hesitation at the pedal. Almost as if the car didn't want to go past 45. It kicked back- I think I would describe it. I didn't try to force it, so I just got off the highway and parked it. What could the lack of power be contributed by? Timing chain? 0r off timing at distributer? Or could it be something tO do with the transmission? Thanks for your help.

 

Try a complete tune up. Spark plugs, wires, cap 'n rotor, ect...

 

jumper the aldl port and see if there re any codes flashed

 

1: start a new thread

2: you need fuel, air and spark and the ability to exhale

so, is the intake free and clear?
any clogs in the exhaust path
are the fuel pump and injectors working properly? was the vehicle sitting for a while? How grotesque was the fuel when you changed the fuel filter (there's a hint in there by the way)

put vehicle in 1st gear and run up to 4k rpm's, does it miss?

is the problem speed related or engine speed related?

start a new thread

 

Nope, none

 

Everything is new (if you're answering my thread)

 

I was getting the same symptoms with my '82 Trans Am that has Crossfire TBI. It would hesitate/surge around 1500 RPM under light throttle and was fine above 2000 RPM. Under WOT it would pull to 5500 RPM with no issues.

I had a GM TECH 1 scanner hooked up to the ALDL port and the MAP sensor didn't show any abnormalities but I decided to change it out since it was 29 years old. The hesitation/surge around 1500 RPM is GONE! The old MAP sensor was "working" but I think it wasn't as sensitive as it should be anymore. With the new MAP the throttle response also seems more crisp.

 

They say MAPs are vehicle specific, did you get one for your exact car or just for similar model?

 

The MAP is an AC Delco replacement. The part number cross references to other GM vehicles made from '82-04 so they must be somewhat universal. The MAP senses pressure changes in the intake manifold from the throttle blades opening and closing.