Lt1 only starts at WOT. Black smoke. I'm stumped.
08-11-2009, 08:07 AM
#1
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Lt1 only starts at WOT. Black smoke. I'm stumped. Help! So my car was just sitting for 6 weeks and now it's starting to run crappy. It's a '95 LT1 with 110K miles and mostly stock. It only starts at WOT and with a puff of black/grey smoke and then will run fine except for when I hit the gas the engine will dip down from say 1500 rpm to 800 and then back up again, which makes it very difficult to shift gears. I've been trying to figure this out for several days with no luck...
New parts:
injector o-ring seals
fuel filter (2k miles)
fuel pressure regulator
Test performed:
FPR ok,
fuel pump volume is 1 quart in 36 seconds
no visible leaks in fuel lines
41 psi when fuel pump is primed
when engine is switched off fuel psi drops from 41 to 0 in 2-3 seconds (check valve?)
32 psi at idle, increases with engine rpm, jumps up and down when revved
42 psi at WOT
resistance (Ohms) across the injectors is the same
all injectors receive battery power when they're supposed to
I unplugged the injectors one at a time with the engine running and they all had a similar effect.
Spark/ignition seems ok but not fully tested.
Before now my car would be slow to start up (extended crank time) but that's probably just the check valve on the fuel pump. But i think my fuel pump might be weak because before at WOT it seemed like my engine would miss as if it wasn't getting enough fuel, but now it runs smoothly and accelerates well under WOT so i'm confused.
I don't see any intake manifold vacuum leaks. My buddy who stored my car said he drove it once, it ran fine until he went uphill with only 2-3 gallons in the gas tank and then it started to run like it does now. So maybe my fuel pump is on its way out?
Any advice is appreciated! I don't want to just throw money at the problem!
New parts:
injector o-ring seals
fuel filter (2k miles)
fuel pressure regulator
Test performed:
FPR ok,
fuel pump volume is 1 quart in 36 seconds
no visible leaks in fuel lines
41 psi when fuel pump is primed
when engine is switched off fuel psi drops from 41 to 0 in 2-3 seconds (check valve?)
32 psi at idle, increases with engine rpm, jumps up and down when revved
42 psi at WOT
resistance (Ohms) across the injectors is the same
all injectors receive battery power when they're supposed to
I unplugged the injectors one at a time with the engine running and they all had a similar effect.
Spark/ignition seems ok but not fully tested.
Before now my car would be slow to start up (extended crank time) but that's probably just the check valve on the fuel pump. But i think my fuel pump might be weak because before at WOT it seemed like my engine would miss as if it wasn't getting enough fuel, but now it runs smoothly and accelerates well under WOT so i'm confused.
I don't see any intake manifold vacuum leaks. My buddy who stored my car said he drove it once, it ran fine until he went uphill with only 2-3 gallons in the gas tank and then it started to run like it does now. So maybe my fuel pump is on its way out?
Any advice is appreciated! I don't want to just throw money at the problem!
08-11-2009, 10:36 AM
#5
Unplug the MAF and see if the car starts. My car ran exactly the same with a shorted out MAF sensor. With the MAF unpluged the car would start and run OK in Speed Density. With the maf pluged in I had to damn near floor the car to keep it running, and black smoke would pour our the back.
08-11-2009, 12:10 PM
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I've started it with the MAF unplugged and plugged and it didn't seem to make a difference. Without the throttle wide open it will crank and "putt-putt" a couple of times and that's it. From what i've heard starting it at WOT puts the PCM in Flood mode and cuts off the fuel. I'll check my injectors for leaks as soon as i get a chance. Is there a way to clean/test the MAF?
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08-11-2009, 10:59 PM
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Yeah at first i was sure it was the pump but after testing the pressure it seems to be ok. I'll see if i can get my hands on a vacuum gauge tomorrow to test the intake manifold vacuum.
08-11-2009, 11:52 PM
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It definitely sounds like it's getting too much fuel. The reason it starts at WOT is because the car is in flood clear mode and shuts off the injectors when the TPS is at wide open while cranking. Check the coolant temp. sensor (the one in the waterpump). If the sensor has failed and the PCM thinks it's really cold out it will dump extra fuel.
08-12-2009, 07:03 AM
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It definitely sounds like it's getting too much fuel. The reason it starts at WOT is because the car is in flood clear mode and shuts off the injectors when the TPS is at wide open while cranking. Check the coolant temp. sensor (the one in the waterpump). If the sensor has failed and the PCM thinks it's really cold out it will dump extra fuel.
