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6.2 surging in off throttle(yes I know technically its not a throttle) decelleration

B

boneheaddoctor

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Location
Northern Virginia, USA
Here's my question, been searching for days without finding anything similar.

Truck starts and runs well, fairly decent idle occasional hiccup, no smoke. this started after sorting out an air infiltration issue with rubber hoses and old fuel filter. At least I started noticing it then. Runs better now than it has in the 15 years I've owned it.

1987 GMC V2500 TH400 tans, 4.10 gears 33inch tires, 3/4 ton 4X4 runs well but taking foot off of the go pedal at higher speeds (say 50 to 70 is where you notice it most through engine braking) the first few seconds there is a notable surging before it tapers off and stops.

While I was replacing the rubber hoses and fuel filter I also installed a low pressure electric fuel pump on the engine side of the 6 port valve as getting the engine running after a fuel filter replacement was an exercise in futility and always needed a second vehicle and jumper cables and about 20 minutes of trying before it ever ran again. So it would self prime. That resolved the hard starting and rough idle I'd suffered with for years.

Tuck only has 74,XXX actual original miles on it. Any ideas...I'm completely stumped.
 
No idea what the fuel pressure is under the hood. The electric pump is rated at 9-14 psi back by the 6 port valve. I've got to figure out where to tap into a line to measure it up under the hood. Filter is only a few weeks old, and runs really well, EGT's are in check (yes I have one since I installed a Banks stinger kit over a decade ago).

Just a 3 or 4 quick surges in about 2 or 3 seconds immediately after lifting the foot off the go pedal then it stops on it s own.
 
This sounds like a governor issue in the IP

Does it ever do it at low road speeds, or the engine die ???

A worn governor will cause this sort of surging.


Missy
 
Nope...never does it any other time.....only at higher speeds (over 50) if you suddenly take your foot off the go pedal. There really isn't the same level of "engine braking" (for lack of a better term) at slower speeds Its not a whip your head back and forth type surge, but one you notice if you pay attention (feel more than hear), though the droning from the intake (again banks stinger system that removed the resonator) not to mention the howling of a set of Yokohama Geolander 33" Mud tires basically drowns out the audible part enough to keep you from hearing it, No idea of engine RPM's its occurring at, but figure a TH400, 4.10 gears and 32-33" tires and you have a ballpark at 50-70mph when it happens.

I swear though you can just about feel it in the go pedal if you suddenly back off but keep your foot on it firmly enough.

I never really noticed it before I replaced most of the rubber fuel lines, fuel liter and put the electric pump on it about a week ago (because its almost impossible to get the system bled and running after a fuel filter replacement without it). The fuel is also turned up a bit (just under 1/4 turn but with my driving the EGT's are still in spec as I don't tow heavy loads) and I need significantly less go pedal to keep up with traffic now.

TO be fair...it also runs SIGNIFICANTLY better with a lot more power than before. So maybe its just been hidden by the other recently corrected problems.

Its something I can live with if its not a big issue or something to worry about...as its not extreme, and wife never even noticed it riding on the passenger side.
 
Dang post locked while editing....

No surging at idle....save for the occasional random misfire it would be a smooth idle. Engine never dies...not yet anyway.

Entire truck only has 74,000 and some miles on it....I'm the second owner, Mileage verified as actual and true before I bought it by the County Garage Maintenance records(I called them and actually talked to the guy who maintained it based on the VIN number records) as it was owned by the county before it was sold due to age.

It was used as a plow truck at a courthouse parking lot, and then sat a lot at a Fire Dept. (couple years) who never really used it before it went to auction and the dealer I got it from won it.
 
ak diesel driver said:
IIRC db2s are supposed to be max 8-9 psi
Click to expand...

Yup. AK nailed it. Over 9 psi creates too high pressure in the ip case and ramps up the timing. If you are running higher rpm then the demand slows or almost stops suddenly, the pressure will go up in the case and advance the timing at a time when retarding it would be appropriate. Don't think added fuel into the cylinder, the fuel amount to the injector is correct. The timing is off because the pressure is too high.

http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html Put a "T" inline before the ip then hook to the gauge. run the line under the wiper blade to see through windshield while driving. If you want permanent gauge in the cab even better, but get one that shows vacuum like this one for diagnostics.
 
My old GMC 6.5 TD would do that actually. Surge a few times after lifting your foot off the pedal. Eventually the crankshaft position sensor showed itself as the issue. Timing I'd assume was what caused the surge.
 
I just ordered a new lower pressure pump last night (4-6 psi pump) It should be here Thursday or Friday and I've got a lot of other stuff to do before then so don't have time to see what it actually measures at, but I can assume its way too high now if the current one puts anything near what its rated at out. I seriously appreciate the input everyone.

No I don't drive it a lot...so hopefully no harm will be done before its replaced.
 
Ok, been meaning to update this the last couple nights. Got the new electric pump in, the one that is 4-7 psi instead of 9-14 psi. And as AK diesel driver guessed. The problem went away, so it was the excessive pressure doing it. Many thanks. Saved me a lot of worrying and probibly eventual damage.
 
When you get the time in the future, adding a permanent gauge will pay for itself. The ip will pull fuel passed a non working LP (sound does not mean its pumping) the added load on the IP will wear it out quicker. Then you also have the benifit of real time diagnostics. Just something to keep in mind.
 
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