Hesitation Problems 99gmc 5.3L

Need help since GM can't seem to diagnose problem.I had my 99 gmc in numerous times to the dealer and they keep soaking me money for things that haven't cleared the problem up.They changed wires,plugs,did complete fuel injector cleaning,changed fuel filter,cleaned throttle body,put a chemical in the tank.So the problem is i am getting no codes,when stopping vehicle for 5mins then starting again and accelerating it has a bog,a big bog where you can floor it and it spudders and jerks and then it levels out and is fine.ANY SUGGESTIONS?Need a know it all for this one because the experienced trained mechanics at Gm can't fix this one...........Thanks

How many miles are on your truck. Has the fuel pump been replaced?
Accurate (constant) fuel pressure is important to the computer which is responsible for mixing the fuel ratios.
I'd start by reseting the computer and retraining it, by driving it some, using different throttle positions, and hitting passing gear. If it don't clear up, check out the fuel pump.

Hi,the truck has 129000km's on it and the computer has been reflashed twice,i have tried taking battery cable off a few times,it clears it up for about a day then back to the same ole problem.Is there a way to check to see if the fuel pump is going?I read some where on the net that a guy changed his temp sensor and it cleared the problem up.Is this sensor expensive?And is there just the one.Because it seems now days on the newer vehicles there is a sensor for everything.And if i start changing sensors this will get pretty expensive.I'm looking for a simple but inexpensive fix.Hopefully someone will be able to help me out,this has been going on for a year now/So if the fuel pump was failing wouldn't it have crapped out by now?

I still suspect the fuel pump.It seems practical that with 129000km it's due for replacement if your planning to keep the truck. As a GM service advisor, I would recommend this to my business customers,as breakdowns and tow bills are not an option. I purchased a new MU1089 fuel pump on eBay for $120 shipped. I believe your truck will be the same part number. Local labor for installation was quoted to be $175, so I installed it myself. Lifting and sliding the pickup box rearward is the best way to install if at all possible.
Again I'd recommend this repair as preventative maintainence. You'll be miles ahead and it will probably fix the problem.

A FP pressure test should be done to avoid guess work here, 65 psi on cranking.

i've seen a couple folks with a trans. case (module?) prob that
was making the trans think it was in 4lo & was shifting right from 1st to 4th.
It's a longshot and I cant find where I saw that again but I thought it was
worth mentioning. GOOD LUCK

A FP pressure test should be done to avoid guess work here, 65 psi on cranking.
I agree max, but even when it might test good, the mileage tells me that intermitant problems can be occuring (expected) within the pump. Its 8 years old, and these pumps have a history of going bad. Check valve, low pressure, backfire ect. I'd put my money on the pump and not the sensors. Tell me how it turns out sierra99.

Hi,Regalwhiskey,the fuelpump was diagnosed as being fine.What i don't understand is GM called me at work and said it was putting out 50psi.But a total pressure of 57psi was the highest.GM then tells me that the pressure was misdiagnosed and the vent selenoid and cannister have to be replaced.Something about being plugged.Would this cause the problem? Then filling up the tank then test driving it and seeing if hesitation is still there.They are wondering about tank being full/half/1/4 to be sure.I said the hesitation is there full tank or not but,That will cost 546.00 installed. or if by chance it is the fuel pump 818.00.installed I can't spend this much money on an assumption.Any ideas what i should do?I am going to try taking the gas cap off when it acts up to relieve the pressure if there is any to see if the selenoid is creating a vacuum that may be creating this problem. Will keep ya posted thanks.

I tried the taking off the gas cap trick it helps but doesn't eliminate the problem.Any suggestions??

I don't understand this, 50 psi is not good. The cannister vent issue would set a code. Whatever they do/recommned make sure they guarantee this will repair it.

Good news guys,after further extensive diagonstics the problem was the Mass Airflow sensor.Problem fixed yey:) Thanks for all the input guys and will definitley be back if another problem stumps me.

My 99 is doing that exact thing, how did you come to find that out, did it show up on a system scan?

Wow. I stumbled across this thread. My exact Sierra has been doing the same thing for two years now. Start it up cold or hot and it has zero power even when stomped to the floor for about 1/4 of a mile. Then after that the gremlins work out of it and it runs like a champ until you turn it off again. I had the fuel filter and fuel pump replaced. No change. No codes come up. I even cleaned the MAF. Do you think my problem would be solved by replacing the MAF? Or is it just a crap shoot?

There is no dealer where I live (have to ship the vehicles in by air or barge). It's up to me or the local independent mechanics to solve the mystery. Forum help is appreciated and welcomed.

Thanks.

Has a oiled air filter been used on these vehicles? A K&N for instance?

See below.....



Info - Automatic Transmission Shift, Engine Driveability Concerns or Service Engine Soon (SES) Light On as a Result of the Use of an Excessively/Over-Oiled Aftermarket, Reusable Air Filter #04-07-30-013A - (Jan 25, 2005)

2005 and Prior GM Cars and Light Duty Trucks

2005 and Prior Saturn Models

2003-2005 HUMMER H2

This bulletin is being revised to add additional model years and to clarify warranty coverage. Please discard Corporate Bulletin Number 04-07-30-013 (Section 07 -- Transmission/Transaxle).

DO THIS:

First, Inspect the vehicle for a reusable aftermarket excessively over- oiled air filter.

DON'T DO THIS:

DO NOT repair MAF sensors under warranty if concerns result from the use of an excessively/over-oiled aftermarket, reusable air filter.

The use of an excessively/over-oiled aftermarket, reusable air filter may result in:

• Service Engine Soon (SES) Light On
• Transmission shift concerns, slipping and damaged clutch(es) or band(s)
• Engine driveability concerns, poor acceleration from a stop, limited engine RPM range

The oil that is used on these air filter elements may be transferred onto the Mass Air Flow (MAF) sensor causing contamination of the sensor. As a result, the Grams per Second (GPS) signal from the MAF may be low and any or all of the concerns listed above may occur.

When servicing a vehicle with any of these concerns, be sure to check for the presence of an aftermarket reusable, excessively/over-oiled air filter. The MAF, GPS reading should be compared to a like vehicle with an OEM air box and filter under the same driving conditions to verify the concern.

The use of an aftermarket reusable air filter DOES NOT void the vehicle's warranty.

If an aftermarket reusable air filter is used, technicians should inspect the MAF sensor element and the air induction hose for contamination of oil prior to making warranty repairs.

Transmission or engine driveability concerns (related to the MAF sensor being contaminated with oil) that are the result of the use of an aftermarket reusable, excessively/over-oiled air filter are not considered to be warrantable repair items.

The airfilter has been changed once. I placed in a FRAM replacement, just a paper one like the OEM. The vehicle has 68,000 miles on it.

Is the MAF seem to be the only problem?? ive chg the pump, pressure regulator(it was low at idle and now fixed but not checked under load yet), filters chg, had a K&N and removed it 2 years ago cleaned maf and seemed to fix it for a while. thought it was the cats but was talked out of chg them. keep getting 137 and 157 codes along with the hessitation like everyoen else. lots of pinging on med to heavy loads. chg one o2 sensor to see if it will only affect one bank. Anybody chg knock sensors with results???

totally getting fed up but want truck to run like it did when new........

Check fp under load, scan it and look at o2 sensor activity also. The engine is lean of course but why, try a bottle of Chevron Techron injector cleaner also. Make sure there are no vacuum or intake leaks this can cause the lean codes also.

I'll let you know what i find. I only have a code reader/eraser don't have one that can watch live time activity of any sensors, guess that would be like a good snap-on style one ey?? can i check vacume leak like i would on old school v8, meaning spray carb cleaner or ether around edge of intake to watch for rpm chg, cant hear any hoses leaking. are intake leaks common on the 5.3L.

Yes they can be any, air entering behind the maf can cause this.

well FP is good, about 60-64psi under load and driving. doesnt seem to be any noticeable vacuum leak, even pulled off a hose to make more sound diff, but nothing, sprayed intake all over and MA but no idle chg.
does this start leading back to blocked cats??? or worth just chg the intake gaskets???

Check the cats, disconnect the pipes from the manifolds if possible.

not meaning to ask a stupid question but take cat's off, unbolt the pipes and then what, run it open exhaust or look down pipes?? will i see if they are blocked???

Run it, if it is ok the cat is an issue, you will not see anything.

I just had a cat test done on my 99 (5.7L), they drilled holes in the pipe before the cats and checked backpressure. After the backbressure test they welded the holes shut...