Hesitation/surging at @1500-2000 RPM light load

TGUNDO2003

MEMBER
1996 GMC SUBURBAN
5.7L
V8
2WD
AUTOMATIC
250,000 MILES

@ 1500-2000 RPM under light load there is hesitition going less than 30MPH and surging going more than 30. If I give it more gas, let off the gas, or run any other RPM it purrs like a kitten and drives great. No problems starting, idles very smooth. At first I was thinking tuneup, but now I am wondering if it might be a transmission/torque converter issue. Orignal tranny on it, overdue for a fluid/filter change. I replaced a failed EGR valve about a month before this started happening, not sure if that could be related.

4 Replies

JACOBANDNICKOLAS

MECHANIC
108,117 POSTS

Has the check engine light come on? Is the fuel pump pressure within the manufacturer's specs?

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Friday, April 4th, 2014 AT 7:04 PM

TGUNDO2003

MEMBER
5 POSTS

Thanks for the reply. It's 18 years old and 250K miles. The SES light is always on!

Seriously- it is always on because I have a p0300 "catalytic system below efficiency threshold" code all the time, and I live where there is no emissions testing so I have not fixed that yet due to lack of funds. I think I have 2 O2 sensors that are flaky (O21/2 and O22/2), they don't graph out the same as the other two during drives, not as consistent "up and down" on the graphs and the p0300 is intermittent, I can clear it and go a week before it comes back. It also might need a new cat on that side, maybe the sensors are right. IDK. Its been that way a long time and has never seemed to affect the driving.

That being said, I cleared it back out, and I get a P1406 code, which is the EGR valve. That code came back within 20 minutes of clearing it (I also drove it to make the hesitation/surging happen on purpose). I am beginning to wonder if the replacement EGR is flaky. The one I replaced was the original, it was really carboned up, cleaning it only lasted a couple days. It was getting stuck, the truck threw that same code and wouldn't idle worth a salt. After replacement it idles great, just the intermittent surging/hesitation at certain speeds/RPM.

I have not had a chance to put the fuel pressure gauge on it yet- I will try to get that done this afternoon. The fuel pump and filter are about a year or so old, @25K miles.

Thanks!

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Saturday, April 5th, 2014 AT 9:00 AM

JACOBANDNICKOLAS

MECHANIC
108,117 POSTS

Was the intake carboned up where the EGR is mounted? You know, I worked on a 99 Chevy truck today that have very similar symptoms. I pulled 3 codes, temp sensor / closed circuit, Catalyst below threshold, and an EVAP code. To make a long story short, there are two temp sensors on the vehicle, one for the gauge in the truck and the other sends a signal to the ECM indicating coolant temp. It ended up being that sensor. When the ECM runs the engine in a closed circuit, the computer dumps fuel to the engine because it thinks it is cold. This one thought it was -40 degrees. I replaced that sensor (15.00 part) and cleared the codes. It ran perfect after that.

The reason I mention that is because it had very similar symptoms and I was just wondering if you checked the trouble codes even though the light has been on for a long time.

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Saturday, April 5th, 2014 AT 6:43 PM

TGUNDO2003

MEMBER
5 POSTS

Thanks for the feedback. Only codes its throwing is p0300 and p1406.

The two O2 sensors that are not behaiving normally are on the same side. For a long time it was only the post cat sensor that was mis-behaving so I ignored it thinking it was just a bad O2. Now that the pre-cat one is doing the same thing I am wondering if the cat is getting clogged up. They are the orignal cats on the truck.

The orignal EGR was getting stuck. THe new EGR move as smooth as it should. Could excessive backpressure from a clogged cat cause issues with a properly functioning EGR?

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Sunday, April 6th, 2014 AT 6:58 PM