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95 4.3 CPI surge/cutout under hard accel.


grif_566
06-24-2007, 12:20 AM
Trying to diagnose an odd one.
My buddies truck is acting like it's hitting a rev. limit at 2000rpm under
what you would generally refer to as "normal" acceleration.
If you "baby" it and gradually open the throttle it will run fine but any demand for quicker accel causes a surging cutout (feels like fuel cut).
TPS seems to be ok, i haven't tested the MAP or Ign. Module (Ign. Module was replaced about 3 months ago, failed for unknown reasons).
Although I havent checked fuel psi when this happens I don't suspect
fuel sup[ply is the problem, i can gradually open the throttle and run the engine (smoothly)
all the way up to the rev limit at 5500rpm.
I can make the surge cutout occur at higher rpm also, when you gradually raise the rpm to 3000 and then stab the throttle it begins to surge/cutout at 4000rpm.
For us old guys, it feels alot like a bad accelerator pump/circut in carburetted engines.
Anyone run into this problem???
I'm an experienced mechanic but now work in the marine industry and have no access to chevy tech data, much less the proper diag tools for domestic PCM's..

MT-2500
06-24-2007, 09:22 AM
Welcome to AF
Fuel pressure is very important on that GM V6 engine.
If it does not have full fuel pressure do not leave home without it.
Fuel pressure specs on code w engine for that year is 55/61.
Put a fuel pressure gauge on it and drive it with gauge hooked up and post back fuel pressure readings.
Check fuel filter>
Any check engine lights on or blinking?
How many miles on it and when was last tune up?
Ac Delco plugs?

grif_566
06-24-2007, 04:11 PM
MT-2500

thanks for taking a look.
unfortunatley i didn't have my pressure gauge set w/me this weekend
so i couldn't confirm good or bad, doesn't act like a psi problem though
I am curious what the pai is showing when the cutout occurs.
It doesn't stall the engine, when it occurs if you keep the throttle down it simply continues until you back off say about a 1/4 throttle.
While it occurs, the engine doesn't run hard or rough per say, it just hits 2200 and it sounds as if you just shut off the fuel for a second, it drops to 2000 and hits again but at around 75% throttle under what we would consider normal rate of accelerator opening it jsut plays pickle from 2000 - 2200 or so.
Like the fuel is on then off then on then off, not just one or two injectors though it's just like you took the fuel away and the engine is revving down. Sort of like when you shut down a diesel, think of it like you are engaging and disengaging the fuel cut solenoid, the engine drops rpm when the fuel is cut and comes back when the fuel is there. Keep in mind that when the throttle is opened say at "grandma" rate the engine runs smooth and will
run all the way to 5500 rpm. If I grandma it up to 3000 and then stab the throttle to 75% open it reoccurs at 4000 rpm, back off a little and grandma it and I can get it to 5500.
THe engine has had the cap, rotor, plugs, wires replaced (ac delco only always, i know the value of oe). That came about after a no start condition occurred about 6 months ago.
The no start turned out to be a deteriorated vacuum hose that was hand down and resting on top of the dist cap, it had gotten hot and began to
crack and split causing enough of a vacuum leak to prevent starting.
Onc the hose was repaired all was good.
4 months ago the fuel pump failed, replaced that with oe and at that time all psi's were in spec. (i sure learned a good lesson about this CPI and fuel psi from that job...oh and how picky the PCM is about being cleared too)
Also had the Ign. Module fail at that same time, I'm not sure if it failed right when the fuel pump failed or not as I diagnosed the fuel pump failure without checking for codes (had no check eng light at that time, but found a code for the Ign. module when i couldn't restart the engine after replacing the pump and clearing the PCM). Code went away with the new module.
This current gremlin shows no MIL either but I did not run a code check so I don't know if a code is in there or not. I reset the PCM three times over the weekend with no improvment. Still drivable but you just have to be easy on the throttle.
All has been fine since until just yesterday when he called and said it was cutting out. I went and drove it and found i could drive it just couldn't accel like you normally would want to. The engine doesn't run poorly otherwise and doesn't feel down on power either, and like i was saying you can drive it and if it hits 2200 too quicly I just back off and go a little easier and it gets past that, shifts into all gears too, problem is the cutout mostly occurs at about the same rpm the trans shifts at, he thought it might be a transmission problem but after checking it out, I found it does it in or out of gear.
It a wierd one, I really hope there is someone out there that has had this same issue, but with my luck (i always get the worst job dumped in my lap) it a unique failure that noone has encountered.

Thanks in advance for any other info..

Greg

riptide44
06-25-2007, 12:02 AM
sounds to me like a tps problem ( hitting a dead spot ) did you test it with a voltmeter for both voltage and ohms . however it could still be a fuel problem related to the regulator or injenction unit itself ( very ,very common problem ) .

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