Engine Surging 2008 Truck 5.3L

**rlchv70** · 08-09-2016, 05:48 AM

Sorry if this is the wrong forum, please direct me to the right place if so.

Having difficulty troubleshooting my truck. It is a 2008 GMC Sierra with the 5.3L flex fuel with AFM. Relatively low mileage (~70k).

It has had an issue with surging for a while, but has now gotten worse. It started as just a slightly noticeable "jerk" under steady cruising. Now it is a fairly violent surge while cruising and may drop itself out of cruise control. Speedometer will drop by 10MPH with no noticeable change in speed. It will have a high idle when coming to a stop, and then settle down after a few seconds. No codes or SES.

Troubleshooting done so far:•Changed plugs & wires. No effect.
•Attached ELM327 OBD scanner. Nothing out of the ordinary that I can tell. MAF reading fluctuates with surging. O2 sensors reading normally.
•Changed MAF Sensor. No effect.
•Fuel pressure is good, but fluctuates rapidly (~5psi) at idle and decel, and sometimes when surging.
•Unplugged MAF circuit: One time, this seemed to eliminate the issue. 2nd time, it caused the ECU to enter overheat limp-home mode.
•Checked/wiggled wiring at sensors and ECU. No effect
•Sprayed starting fluid around intake to look for vacuum leak, no effect.
•Found loose connection at battery, replaced terminal. No effect.
Please let me know the next step for troubleshooting, will post results. I'd rather diagnose this properly than throw parts at it.

Thanks.

***SMWS6TA*** · 08-09-2016, 06:14 AM

Clean the MAF sensor with MAF Cleaner, do not touch the wire. Just spray and let dry, repeat a few times.

Are you using a K&N filter?

Might also want to check the Throttle body to see if it's gummed up. Particularly around the IAC.

**rlchv70** · 08-09-2016, 06:18 AM

MAF sensor is new. I also cleaned the old one before replacing it.

Throttle body is clean.

***SMWS6TA*** · 08-09-2016, 06:45 AM

The limp mode is got me thinking to check the alternator or battery.

I had a situation similar in that my ECM went in limp mode trying to charge the battery. It raised my rpms to a constant 2500 rpms, etc...

Turned out the alternator was fried because of a power steering leak.

Might be worth checking into at least to make sure it's not it.

Are you getting any readings with the MAP sensor?

**rlchv70** · 08-09-2016, 06:53 AM

Alternator appears to be functioning normally.

MAP is reading fine.

***SMWS6TA*** · 08-09-2016, 07:06 AM

**Whamhammer** · 08-09-2016, 09:14 AM

Could it be a transmission problem, like a torque converter lock up solenoid?

**rlchv70** · 08-09-2016, 09:23 AM

Possibly, but how do I troubleshoot that?

***SMWS6TA*** · 08-09-2016, 09:53 AM

Your going to have to have it look at from a transmission shop.

Engine wise everything seems to check out.

**pajeff02** · 08-09-2016, 10:06 AM

Why is the fuel pressure fluctuating?

**rlchv70** · 08-09-2016, 12:54 PM

Originally Posted by pajeff02

Why is the fuel pressure fluctuating?

Good question. Any thoughts?

***SMWS6TA*** · 08-09-2016, 01:18 PM

Could be clogged fuel filter, pump &/or regulator going bad. Hell even the sock could be clogged.

**pajeff02** · 08-12-2016, 03:23 AM

Even if the engine is stumbling the fuel pressure at the rail should remain fairly constant.

**0rion** · 08-12-2016, 07:29 AM

I would revisit the vacuum leak......make sure you check all the vacuum lines really good. Symptoms sound a lot like a vacuum leak to me. The high idle is what makes me think that.

***SMWS6TA*** · 08-12-2016, 08:52 AM

Now that Tim mention it....it does remind me about when my intake had an 8" vacuum leak because the rear nuts on the Fast intake backed off. At idle it would surge up and down and wouldn't tune for shit. But once the rpms went up it would run fine.

**bluehawk2000** · 08-14-2016, 02:59 AM

Also check the number 1 and 7 cylinder spark plugs. In our 5.3 we have issues with the oil return system fouling out those two plugs.

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**rlchv70** · 08-17-2016, 02:14 PM

So, took the truck to the dealer to let them diagnose it. They hooked it up to their Tech2 and said that the problem went away when they unlocked the torque converter. They insisted on replacing the entire transmission, so I took it home instead. I bought a new converter and installed it. Problem still persists.

Any thoughts guys? Anyone with access to a Tech2 in the chicagoland area?

Thanks!

**pajeff02** · 08-22-2016, 11:39 AM

So the converter is in lockup all the time?

**Naaman** · 08-22-2016, 07:56 PM

Originally Posted by rlchv70

So, took the truck to the dealer to let them diagnose it. They hooked it up to their Tech2 and said that the problem went away when they unlocked the torque converter. They insisted on replacing the entire transmission, so I took it home instead. I bought a new converter and installed it. Problem still persists.

Any thoughts guys? Anyone with access to a Tech2 in the chicagoland area?

Thanks!

This is the kind of thing that drives me nuts. It's nearly 100% of the time that I take my car to the dealer, I get it back with something new wrong with it. I have resolved to never let a dealer touch my car again. Not even for routine maintenance. If I were in your shoes right now, my mind would be dreaming up all kinds of ways that the dealer might have sabotaged my car in order to guarantee that a simple torque converter swap would not fix the issue. That's how much I don't trust them.

Can't blame you for not pulling the trigger on that one, bud.

With respect to your problem, the only time I have ever experienced a surging issue was when the O-ring on my IAC valve was damaged. It looked fine when installed, but when removed, it was actually split in half (like a bagel cut down the middle, so there were "two" O-rings). After over a week of trying to resolve the issue, turned out to be a $0.15 part and 1 minute of labor that saved the day.