Skipping Hummer h2

[trofeo16]

my h2 was doing the same thing. Between 42 and 55 mph the rpm's would be bouncing. Almost like you were slightly revving the motor. No check engine light no codes. Took it to the hummer dealer. They found an entire gm service belletin on the issue. Turns out there is something in the 4l60e tranny that is excentuated by exhaust,intake,maf spacers..etc. I removed all the after market and it went away for the most part. Replaced parts and it came back. If you ask hummer dealer to do a search on bouncing rpms within the service bulletins you will find the article. Hope that helps.

[H2Much]

I had a similar issue as well, ended up being a small air leak in the intake manifold. Once it was fixed, the problem was gone. Only way to check for it was a mechanic who had a smoke machine testing system. Sometimes the check engine light comes on, sometimes it doesn't. The error code just says that the air and fuel mixture is not right, usually hinting to the mass air flow sensor going bad, but this wasn't the case with mine.

[HUMTECH]

If you have an intake(vacuum) leak it will almost always set a lean mixture code and will turn on the ses light. Need a scan tool to force it into closed loop operation and monitor fuel trim levels while spraying air intake cleaner around the intake gaskets with the engine cold, if the fuel trim levels change dramatically the intake is leaking. Dont think this is your issue though. I will put some thought into it and see what i can find.

[HUMTECH]

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
<A href="http://gsi.xw.gm.com/si/showDoc.do?docSyskey=1899786&from=sm#ss1-1899786">Condition/Concern:

99 through 05 GEN III truck engines may experience any of the following:

1. <LI type=1>A cold rough idle that diminishes as the engine warms up. <LI type=1>Possible DTCS P0171, P0174, or P0300.
2. Random misfires at idle.

<A href="http://gsi.xw.gm.com/si/showDoc.do?docSyskey=1899786&from=sm#ss2-1899786">Recommendation/Instructions:

The intake gasket seals may be leaking around the intake ports, causing a lean condition. The intake gasket seals may leak around the bottom side of the port, making it a challenge to detect the vacuum leak. The technician may also notice lower than normal MAF readings if the intake seals are leaking.

1. <LI type=1>If leaking intake gasket seals are found, replace the intake gaskets. During the repair be sure to use the current GMSPO part number for replacement intake gaskets. The current GMSPO part number for intake gaskets will have a teal green intake gasket seal. The old version of the intake gaskets were orange in color. Use only the teal green colored gaskets for a repair.
2. The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. If the plenum is found to be warped it will need to be replaced. Service Information states, "An intake manifold with warpage in excess of 3 mm (0.118 in) over a 200 mm (7.87 in) area should be replaced". Intake warpage is measured at the gasket sealing surfaces, and should be measured within a 200 mm (7.87 in) area. This measurement is across only two of the intake runner port openings. Measurements taken across the entire distance of all four intake runner ports will lead to unnecessary manifold replacements.