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Old 08-01-2007, 05:13 PM   #1
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god d@mn gm

I got a 91 corvette in the shop.. this thing will not dile for crap once it warms up. its got 42 psi of fuel pressure on a hot soak and 32 while running. it has a oem distributor new fuel filter, plugs and wires, and even a new crank sensor.. still running like crap.. exhaust isn't clogged. mass air flow is working, iac working fine baros good ect is good the only thing I got to go on at this point is when you unplug one injector on either bank it smooths out.. it has two injector driver modules on two seperate circuits. they don't even share a common ground which I checked on both. I am at my whits end on this thing I really want to unplug an injector and give it back to the customer. please help

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Old 08-01-2007, 07:00 PM   #2
I HAVE A BELLYBUTTON!

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3 blocks of C4
50 feet of det cord
3 big bottles of propane
1 video camera
That's a start. Don't know if it'll help your customer, but I think it would at least alleviate some of your stress. I added the video camera so you could share it with all of us.

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Old 08-01-2007, 07:46 PM   #3
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I know you said it was a 91, but by any chance is it a late 91 with an LT1? In other words, does the POS have Opti-Spark? If so, thats your problem!!!

Seriously, you know what your doing dude. Your not a moron. This is not OBDII, so you do NOT have a complex computer adjusting for the missing injector. If you drop an injector and it SMOOTHS, you only have a few more things to check.

Does is smooth out when you drop an injector from the OTHER driver? The one driver could possibly be bad, and you other looked the dual driver when testing? Is 32 PSI running spec? That could be right, I don't have access to alldata anymore, but that seems a little low (off the top of my head after a long day of work at 8:45 pm..)for a V8 at idle.

Just for shits and giggles, did you get some propane and check for a vacumme leak?

What did the compression test show? Maybe the motor is just dog tired and wore out, and dropping a cylinder is balancing out the motor? I dunno, its a long shot, but it looks like you need one right about now..

Tell your master tech to handle it
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Old 08-01-2007, 08:26 PM   #4
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Hey dude, I just saw your phone call from 5:15. My bad dude. I have the top/doors off the Jeep, must have not heard it ring. I went straight to blockbuster, grabbed a movie, watched it with Kim, and then jumped on here before bed. I haven't even looked at my phone since like 4:00 pm Give me a call tomorrow, and let me know whats going on. I am not ignoring you, just tired
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Old 08-01-2007, 08:43 PM   #5
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ok first of all it is not an opitspark system it is an early 91 with the crap injectors. It does have computer comtrolled fuel, however it has had a laundry list of parts thrown at it already.ranging from fuel pump to distributor all the way down to the lowly ecu and fuel filter. I checked timing and it didn't jump time. I have between 115 and 135 on all cylinders when doing compression. I am fine on a leak down test and I did test exhaust back pressure out of flustration.. I am at whits end with this thing I am going to put the fuel pump on the scope tommorow so I can watch the signal the power and the ground all at once, I am hoping for either a loss of power or signal.. and to awnser your give it to the master tech. he gave up already and threatened to roll his box out the door if he hadto touch it again.. anyone have a camera I can borrow I think we may buy this one to jump it
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Old 08-01-2007, 09:38 PM   #6
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I remember we replace alot of TPS's on them. It shouldn't smooth out by dropping injecter signal though. Cyl bal test reveal anything? I'd think manifold gasket sucking air, maybe from the valley side is why it hasn't been found yet. Knowbody wants to pull it just to see if it has oil residue crossing over.
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Old 08-01-2007, 09:44 PM   #7
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If it runs like crap after it warms up I'd be guessing the O2 sensors... I know it doesn't have an OBDII port but it should have an OBD port under the dash somewhere. I can't remember the two holes, but if you jump them with a piece of wire it should flash a code sequence in the dash.
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Old 08-01-2007, 09:57 PM   #8
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Do you have a scan tool that can command it into open loop? Maybe try throwing it into open loop and see if it smooths out. I'm thinking that if it runs smooth until it warms up, then starts misfiring when it goes into closed loop, then you could have something along the line of a skewed O2 sensor, or a wiring problem that is skewing the O2 voltage. Seems like I remember one of the more critical ECM grounds being up there around either the thermostat housing or the front of the intake on a lot of GM V8s. I've run into a few cases over the years where a skewed Oxygen sensor caused a misfire. Pretty rare, but I have seen it on more than one occasion.

Some other oddball stuff that comes to mind...

Lump of carbon stuck under the EGR valve pintle?
Injector leak test? Injector balance test?
I can also remember a lot of late 80s-early 90s GM multi port FI systems having problems with windings inside the injectors shorting. Strong chemicals such as injector cleaners would erode the insulation between the windings in the injectors, causing the windings to short, but it would only happen under load. The resistance would check fine, but the windings would pull together and short when the injector was triggered. We used to have to do an amp draw test on each injector, using a light bulb in the test circuit as a current limiter.

Hope this is of some help. Nothing worse than having a problem car eat your ass alive. My worst experience so far in my 25 years of doing this stuff was an ambulance with intermittent electrical problems that trampled on my paychecks for nearly 3 months.
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Old 08-02-2007, 05:49 AM   #9
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Dave, come to my house and you can borrow any camera you like. I just want to be there when you jump it...

If nothing else in this thread works, just replace the injectors and see if that helps any.
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Old 08-02-2007, 10:30 AM   #10
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Dumb Question But "did you replace coolant temp sensor?" One of my techs sugested that may be the problem.

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Old 08-02-2007, 07:35 PM   #11
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holy crap what a pain.. ok it wasn't the ect. the timming was right on the money for where it was the harmonic balancer had shifted and was causing a rough idle add to that the fact it had a batch of bad gas you have a mad penguin. but hey its fixed
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Old 08-02-2007, 07:44 PM   #12
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Old 08-03-2007, 10:29 AM   #13
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Old 08-03-2007, 12:56 PM   #14
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I heard they hear voices.....
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Old 08-03-2007, 01:23 PM   #15
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I bet there was a whole new vocabulary developed in that shop!

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