Strange Drivabilty Issue

M

musclecar neal

Engine Builder
Joined
Sep 29, 2002
We are trying to figure out a strange issue we are having.


The car runs fine while just driving around with no boost. When you floor it the car will almost die. You can hold the throttle to the floor for a few seconds and the car seems like it is not even running, then will fire about 2 or 3 times and puke black smoke. It will not rpm at all while it is doing this. The car feels like the engine is off except for the 2 or 3 puffs of black smoke. If you get off the throttle and go back to idle the car will idle fine. If you play with the throttle and get it into boost , once the car has boost it runs fine and pulls like a freight train. We have done plenty of testing with no problems found so far. I was hoping it would be something really stupid but this is getting really dumb now.

Here is a list of testing we have done.

ECM has been tested and runs fine in another car.
Chip works fine in another car.
T+ works fine in another car
All grounds appear fine
plenty of fuel pressure at all times ( in car FP gauge)
tried known good ignition module and coil pack
disconnected cam sensor
tried known good MAF
TPS is set correctly and has no issues
fresh plugs
fresh denso O2 sensor


I appreciate the help I have received from some of the best minds in the Buick world and we are still at the same place we started at. The car is a monster once its up on boost and really runs well then. Its the place between no boost and boost that is killing us. The combo is in my sig. If anyone can give us anymore ideas we would love to hear what you think. I will post the results and will buy the person that comes up with the fix a beer or dinner of choice if I am ever lucky enough to shake their hand. Please don't be sacred to give us some ideas. We need your help.

Thanks for reading this and tell us what you think.

Neal
 
Hum!

Wwe use to have a old chevelle that did the same thing!!! Under normal driving it was fine but when you would floor it the thing would just rev an nothing, but when you would gently mash the throttle untill WOT it would go fine.. Turns out it was the carbureator that was bad... You say you have constant PSI so that makes me think the pump is good. I am sure you have a hot wire going to the pump, I heard a guy was having a problem like this when his hotwire got a kink from the frony control arm an had a similar problem.... I know you said you switched the MAF with a known good one but just for the hell of it disconnect the MAF an try it @ WOT, you will get a SES light of course but its nothing cause of the disconnection.. also a bad plug wire can be the culprit, a buddys car would mis fire @ hi rpm an it was this... Let us know!!!
 
How did this start?

Suddenly? Gradually? Right after some work? I have more questions, but the answers to these might turn on some light bulbs. Possible bad plug wires, possible bad crank sensor. :cool:
 
Brand new tested plug wires, brand new tested crank sensor.

Has been this way since the new engine install.
 
That's a tough one, so many possiblities

I would verify all grounds, especially engine to chassis, and battery to body. By this I mean scrape, star washer, run extras, whatever you need to do to be sure this isn't it. Years ago, I had an engine install drive me nuts cuz it was doing something almost identical. Turned out to be a broken ground wire from the ECM to one of the block grounds. I found it by pulling on it, and it stretched. Sometimes all it takes is one missed wire to cause a lot of headaches. I know you said the TPS has no issues, but have you done a sweep test? :cool:
 
Sounds like your fuel pressure regulator might be sticking giving you to much pressure off idle. Might be drowning the motor.
 
it sounds like a ground problem,did you put the ground wires on passenger side of head back to the head not to the intake???make sure those wires are tight to the back of the head,are the two small screws under module 7mm tight if not big problems,are the mounting bolts for the module braket to intake tight,try all these things,the problem is a ground problem let me know if you have luck.tom
 
I'm not sure if that was meant to be a serious question

rickyracer, what does the fuel pressure regulator do?
Click to expand...

In case it was, I'll answer seriously. The fuel pressure regulator maintains fuel pressure by bypassing pressure above it's setting back to the tank. It's setting is modulated by manifold absolute pressure. In my experience, either a regulator works, or it doesn't. Usually when it doesn't, the diaphragm has ruptured, and it will leak fuel under pressure into the manifold pressure (vacuum) hose, causing an extremely rich idle, and often a no start condition due to gas fouling the plugs. :cool:
 
Originally posted by musclecar neal


TPS is set correctly and has no issues
Click to expand...

I had a bad one, that did the same thing, and it was the TPS, and it tested and looked fine on the scan tools.
 
I notice you have an alcohol injection system. Could you be getting too much alcohol (or too soon) and drowning the spark at relatively low RPM?
 
Not to threadjack here, but I just started having a very similar problem. If I ease into the throttle it runs very smooth and pulls hard, but if I punch it off idle it hesitates/dies for a short moment(reminds me of when accelerator pump in my carburated truck went) and I usually hear a pop through the air filter:confused: This started happening after I shortened my wastegate rod, turned up the smc alky, and installed my new chip. I dont beleive the alcohol is drowning the motor because it doesnt start spraying untill there is 12 psi and all this happens long before that. I put the old chip back in and the same thing happened when it didnt before with that chip. I just put in new plugs the other day gapped at .034 and the wires are one year old magnecor. My combo is in the sig. and here are my idle sm readings with the new chip: TPS: .46, Iac: 12, AF: 06, LB: 36, Int: 128(was bouncing around in the lower 120s with old chip), bL: 129(was 132 before i disconnected and recconected the ecm power), rpm: 775(always idles smooth). Any ideas?

Kyle
 
Kyle,

Mine did that once - cause was the vacuum/boost reference hose to the fuel pressure regulator had blown off, and fuel # wasn't being increased as boost built...

Neal,

Since you're belching black smoke, that's prolly not your answer.
 
Checked all the vaccum hoses, no leaks, grounds are all good. Could it be the cam or crank sensor? Where are they/what do they look like? How do I determine if they are bad? Could it just be way too rich before the boost comes on causing the stumble/backfire?:confused:
 
Drivability issues

The cam sensor is on top of the front cover slightly to the right. (black cylinder with wires coming out). The Crank sensor is right of your crank pulley stradling your harmonic balancer. Any of these sensors can cause this issue. I wuld seriously double and tripple check that TPS. Put a voltmeter on it and slowly increase and decrese throttle. It should be a very steady voltage run. If your cam sensor fails, the vehicle will not be able to be restarted. It will run in "limp home mode" until you shut down. When the crank sensor goes, so does all hope of moving the car short of a towtruck or spare sensor. Many of us keep an extra of both on the glovebox or trunk. I have seen stuck wastegates cause this problem too. Fine tooth comb everything again. Try accelerating gradually and see if it does the same.



87' GN
No AC/PS/AM/FM, BGC Intake, mod intercooler, Alum Rad., Aeromotive A1000, 50 lb/hr inj, 70 mm TB, TE51, stock headers, Adj, FP reg, 3K Orange Stripe, and no more money.
 
Alright, I'll throw my hat in here...

1. Neal, how does the car idle? Does it cold-start properly, and settle into an acceptable idle pattern once it reaches Closed Loop? If so, that typically rules out a lot of things.

2. Other than this issue, is the car's driveability generally good?

3. How does the car react to the following:
a: Quick/Slow Idle to PART-throttle tip-in?
b: [Very Slow/Slow/Medium/Quick] but LINEAR sweeps from Idle to WOT? (We know what it does when you mash the throttle.) ;)

4. Just grasping at straws here, but...do you have a trunk-mounted battery? If so, how have you cabled the car to replace the battery's negative post-to-engine connection?

The fact that it runs well at a slowly-approached WOT leads me to believe that it's probably not one of the usual devices, but something less obvious.

Do you by any chance have Direct Scan recordings for the car? If so, let's take a look at them; I'm especially interested in data for question 3.

Quickwrench@Runbox.com
 
Thats acting like a Cam sensor. Pull the cap off and check that the disc hasnt moved. You really have to grab a good hold of it cause some times it feels fine but its not.

You can also start the car, unplug the cam senso and make a run. see if t cleans up some what.
 
You must log in or register to reply here.

Similar threads

D
runs with stock ecm, not with xfi 2.0
2
Replies
20
Views
845
DGC
D
SlowGrandNational
Small exhaust leaks causing this much headache?
Replies
5
Views
444
SlowGrandNational
SlowGrandNational
S
Werid spark issue
Replies
9
Views
525
THensley
T
H
GREMLIN CONTINUES TO BE PRESENT MAKING DRIVING UNSAFE
2
Replies
29
Views
1K
Ttype6
T
S
Running Rich
Replies
14
Views
752
Sevy14
S
Top