2017 GLS550 Turbo A Underboost CEL

[rpaoness]

Got PO299 turbo A underboost code. Cel came on yesterday and I pulled codes this morning. Not troubleshooting yet and I meant to check freeze frame before I reset to see if it recurs, so no useful data yet.

[kevm14]

That's not good. There are a wide range of possible causes from fairly cheap and easy to turbo replacement.

As such you would want to examine in order of severity. Vacuum pump (mainly the check valve), the boost control vacuum harness, boost control solenoid. Would have to do more research.

[rpaoness]

I had no time to really think about it yet, just pulled the code and drove the kids to school. Seemed to make full power - display showed hitting 400+ hp when flooring it and boost went positive (hard to track while dealing with morning traffic). Code also did not reoccur during that drive cycle. Could be sticky-ish control valve and not recur, or it could be something in the air filter (I have had rodents place acorns in cars in the garage before). We'll see.

[rpaoness]

Would be nice if this could wait to fail more significantly until after I get the driveshaft back in the Jeep. Should be tonight if all goes ok. Would suck to be down to the pickup and boxtruck as our only cars. Dropping kids at school in the box truck seems a little over the top.

[kevm14]

I had no time to really think about it yet, just pulled the code and drove the kids to school. Seemed to make full power - display showed hitting 400+ hp when flooring it and boost went positive (hard to track while dealing with morning traffic). Code also did not reoccur during that drive cycle. Could be sticky-ish control valve and not recur, or it could be something in the air filter (I have had rodents place acorns in cars in the garage before). We'll see.

Wastegate not closing all the way would probably impact part throttle boost performance more than WOT.

[kevm14]

Could be sticky-ish control valve and not recur, or it could be something in the air filter (I have had rodents place acorns in cars in the garage before). We'll see.

https://mbworld.org/forums/gl-class-x16 ... ost8856036

Oct 3, 2023 | 02:55 PM
bwaldmer
Junior Member
I recently had a P0299 low turbo boost error that I cleared by replacing the air filters. My GL550 has around 97k miles on it and recently drove from LA to Little Rock, then Dallas back to Little Rock and used no oil. I really like my GL.

Definitely start simple....

[rpaoness]

Yesterday was the first reoccurence. Cleared again and we'll see how soon it returns.

[kevm14]

Probably check air filters again.

I could look up the recommendations in Xentry but there is a wastegate actuation test you can do and watch the rods on both sides. At idle, the rods should not be loose and vibrating around. Rattling rods can indicate a failed check valve at the vacuum pump. You can source them separately but not through Benz. I replaced mine. I also replaced my boost control solenoid. There is also the vacuum harness. If you can reach, pull the vacuum lines off the wastegates and inspect for oil. There should be none. This happens when the vacuum pump check valve fails, and aside from the check valve being required for proper operation, a busted one will allow oil from the vacuum pump to (eventually) travel all the way down to the waste gates and fill them with oil. In severe cases, this will be a symptom. When filled with oil they will not be able to actuate through their range.

But I'm jumping ahead.

[kevm14]

Some resources in Xentry.

[rpaoness]

Code set again, although I did managed to get through emissions check without even a permanent code just before Thanksgiving, so at least I have as long as I'd like to straigthen it out.

Captured some freeze frame data before I reset. Not sure if it means anything.

I then managed to leave the key on overnight and completely killed the battery. 24 hrs on a slow charger and it seems fine at least so far since.