99 Gmc 1500 - Uneven Idle When Cold, Running Rich

k6lor · March 29, 2010

1999 GMC Sierra 1500

185,000 miles

4.8L Gas/Single Cab 8' Bed

Unknown last oil change

Previous cable company fleet vehicle (my personal fleet truck for about a year before being surplussed)

About a month ago I received a SES light about 15 minutes after a cold start. Truck gets driven maybe once every couple weeks.

Drove around that day, took it to Autozone that night, codes came back rich both banks (forgot the exact codes, I can look in the truck again), sales puke advised probably the MAF. Cleaned the MAF sensor with the special "MAF Sensor Cleaner" from Autozone - no change.

On start up it turns right over every time... Sits at a decent idle for a little while then starts to rise/fall uneven erratic idle... This continues on until I guess when I stop driving at the other end of the trip (maybe 10-15 minutes of drive time tops). Exhaust is noticibly run rich all the time - I'm estimating maybe 10 MPG if that. Truck has a 200 lbs utility topper but no other load to speak of.

Considering I purchased this truck for hardly anything as part of our employee purchase program AND have a new baby on the way in the end of May, I need to make whatever testing/repairs as economical as possible to avoid serious issues with the significant other...

Where should I start? What details do I need to provide?

As always, thank you all in advance!

rmck490 · April 1, 2010

One possibility is you may have an air leak (vaacum leak) in your intake system. First check everywhere a vaacum hose connects to the intake and all the hoses for cracks. It may be as simple as a pulled, cracked, or loose vaacum hose on the intake manifold. Make sure the intake tube from the filter to the throttle body is connected correctly and tight on both ends.

If you don't find anything there, try spraying starter fluid around the intake manifold. The idle speed will increase when you find an area allowing air in. You might want to check the torque settings on the intake manifold as well.

Last thing for vaacum leaks could be the intake gaskets. They could just simply be bad and need replacing.

There's probably other possibilities so others may check in with their suggestions.

Good luck

Maxz695 · November 9, 2011

I went through hell trying to figure this out uneven idle running rich. code 44. I changed the intake GSKfuel filter rebuilt the throttle body replaced all plugs wires cap and rotor. Checked the EGR put in a new timing chain it needed anyway. I even got a new computer for it. I cahnged the O2 sensor and nothing seemed to help . I went back to Nappa and complained that the O2 sensor might be defective. He informed me that I should run the engine in diagnostic mode. It would bypass all sensors and he then said if it smooths out then it,s the TEMP SENSOR/ Go figure. I never in a million years would have thought this was the problem. After unplgging the sensor the idle raised considerable and the engine was smooth and steady. I have yet to go get the sensor but I believe I got this one licked finally!!!! I got a code 15 when running diagnostic after unplugging the temp sensor. Code 15 was low voltage on the temp sensor I but didn,t get the code 44 O2 sensor The short from the temp sensor affects the current sent to the TBI fuel injectors causing the ruogh idle. Give it a shot

desrtrat · April 1, 2010

try cleaning your TB. on most fleet trucks/cars it probably never been done..put a can of Sea Foam in the gas tank then change the fuel filter after 2 tank fulls..

good luck..

gmtech4 · April 2, 2010

consider a manual or alldata account and go as far as you can ,the codes have to currant or able to duplicate the ses light ,,

dont throw parts at it

jaredlucky16 · November 15, 2011

Pull the vacuum line of the fuel pressure regulator while its running and see if there is gas in the vacuum line

jro909 · April 1, 2010

maf sensor or o2 sensors maybe...

k6lor · April 2, 2010

try cleaning your TB. on most fleet trucks/cars it probably never been done..put a can of Sea Foam in the gas tank then change the fuel filter after 2 tank fulls..

good luck..

I'll add this to the list of things to start on...

Reference the MAF/o2 sensors: any simply testing for these with a multimeter?

Thanks all for the help - will try and report back.

Maxz695 · November 16, 2011

http://www.facebook....49984081&type=3 <---- Pictures on my Facebook page here. Problem solved. Bought an upper ball joint yesterday from autozone and maxed out my purchases for the free $20.00 credit. I thought for a while and figured I might as well go ahead and get the Distributor pick up coil with the Credit. Cost about $13.99 Today I removed the distributor and pulled it apart cleaned greesed the shaft wire brushed some rust off the cetrifugal and installed the new pickup coil. ( Old one was in BAD Shape!!) Put it back together threw it in the truck stated it up. Removed the vaccum fron the map sensor for a minite or two and replaced the vaccum,. I set the timing and took it for a ride. It,s like a brand new truck!!! I,m SO SO HAPPY . I turned it off restarted the truch and the engine sings like it,s at a MAJOR CONCERT. It sounds like it just came off the showroom floor. This is a 1987 305 mind you. The wrap on the pickup coil was torn exposing the copper wrap to heat etc sending bad electrical impuses to the computer which in turn sent the wrong information to the TBI Injectors as well as other componants. I reccommend trying this if your having this kind of issue. Took about 30 minutes to complete. Goos luck out there.

Maxz695 · November 15, 2011

Pulled all vaccums and rebuilt the throttle body. No Gas in there. I replaced the timing chain after it started doing it. I repaired the burnt fusable link with the proper size wiring which helped some. I tryed a different map sensor.May not be the right one.But works better than my old one Put in a new coolant plug. Revs high unplugged but when plugged in idle too low and uneven at startup. I bought a remanufactured computer and a nother set of prom and calpack chips. No changes. Runs fine after warmup. Because I went to the junkyard and put a different throttle body on it with seemingly no change I don,t beleive it,s the Idle air senssor or the TPS sensor on the TBI. I did notice that when I tryed to time the distributor the other day after replacing the intake manifold gaskets that even though the timing mark was steady it would at times be jumping around which lead me to think maybe the Pick up coil in the distributor may be goin bad. I have replaced the Distributor Module already with a new replacement. I think it is throughing improper voltage, and the computer is not getting the right readings as a result causing this rich mixture. If I start the engine and give it some steady gas for about 2 minutes the engine seems to kick in and jump about 500 RPM at the same gas peal position and the idles fairly wel after that. Any clues? Last reading I got with the new computer was code 13. Oxygen sensor remained ant the same voltage and did not increase with acceleration I think. I,ll double check today

jro909 · April 2, 2010

easiest is to get the pcm scanned or logged to see what its reading..normally either would trip the check engine light if its broke or getting goofy readings

k6lor · April 1, 2010

Anyone have any tips/ideas?

1999 GMC Sierra 1500
185,000 miles

4.8L Gas/Single Cab 8' Bed

Unknown last oil change

Previous cable company fleet vehicle (my personal fleet truck for about a year before being surplussed)

About a month ago I received a SES light about 15 minutes after a cold start. Truck gets driven maybe once every couple weeks.

Drove around that day, took it to Autozone that night, codes came back rich both banks (forgot the exact codes, I can look in the truck again), sales puke advised probably the MAF. Cleaned the MAF sensor with the special "MAF Sensor Cleaner" from Autozone - no change.

On start up it turns right over every time... Sits at a decent idle for a little while then starts to rise/fall uneven erratic idle... This continues on until I guess when I stop driving at the other end of the trip (maybe 10-15 minutes of drive time tops). Exhaust is noticibly run rich all the time - I'm estimating maybe 10 MPG if that. Truck has a 200 lbs utility topper but no other load to speak of.

Considering I purchased this truck for hardly anything as part of our employee purchase program AND have a new baby on the way in the end of May, I need to make whatever testing/repairs as economical as possible to avoid serious issues with the significant other...

Where should I start? What details do I need to provide?

As always, thank you all in advance!