Dreaded Underboost Code

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

All,

Yesterday I replaced my MAP sensor with Bosch 0281006076 due to a MAP code that still came back even after cleaning the sensor.

At the same time, I replaced my thermostat as I suspected it was failed. I replaced it with a Marshal brand unit that I ordered from an ebay seller in Europe, hoping to get the euro-spec temp. This is meant to be just a placeholder until I am able to get one of the housings from Turbo Diesel Freak.

Before doing all of these things, I checked the codes and did not have P0299. Today after driving a bit, I do.

Both sides of the intercooler have Samco hoses. I thought it may have been the ORM so I plugged the MAF back in, reset codes, took it for a drive, no dice.

I don't believe it is a vacuum issue as I still have some level of boost (in fact it feels like the same amount of boost I have had the whole time owning the vehicle) and I do not have any smoke on startup or hard acceleration. From what I've been reading, it seems like a vacuum issue would result in little to no boost.

I have to imagine this has something to do with either the MAP or the new thermostat? It wasn't there until today. I hope someone more experienced can help. I am supposed to take this jeep on a 1500 mile round trip for the coming holiday weekend so this discovery has not been kind to my blood pressure this morning.

Edit to add:

I spent a few hours cleaning for the second time the old MAP and re-installing it, and then removing, inspecting, and cleaning both CAC hoses and the intake hose between the airbox and the turbo, cleared the codes, and then took it for a drive. The MAP code did not come back but the underboost code did. On my scanner the MAP is reading about 32 and change at idle and up to 72.3 on hard acceleration, not sure if those numbers are normal or not.

This is all making me think the underboost condition may perhaps have always been present, and the only reason the code wasn't being thrown is because the MAP sensor was too caked in carbon to get a reliable enough reading to throw the code. This would make sense as I have been getting 17-19MPG since buying the Jeep in January (fuel economy has gradually risen with the ambient temp) and I was blaming the low fuel economy on the thermostat. However if it is actually an issue of underboost, that is a different story altogether.

WWDiesel · **Posted:** Tue May 23, 2023 2:19 pm

Have you for sure ruled out ANY possible vacuum leaks in the turbo boost controls or a problem with the turbo vacuum solenoid valve or modulator valve.
The small plastic tubing going from the turbo modulator valve over to the turbo vane actuator are notorious for having a small crack in them or partly collapsing due to heat. Need to check ALL vacuum hoses and tubing very closely as any leaks or partial stoppages will greatly limit turbo boost performance.

P0299 -- Turbo/super charger – low boost Mechanical fault

Next possibility; with the engine off, check the stroke of the turbo mechanical vanes by manually operating the actuator with the vacuum line disconnected.
Several owners have reported in the past of finding their variable vanes froze up or partially froze up. If this condition is found, you can try spraying the linkages and vanes from the outside of the turbo with a good penetrating oil like PB Blaster or Kroil and manually stroking the vanes to obtain full travel. But in most cases it does require the removal of the turbocharger and opening it up and physically cleaning the vane operator mechanism. (Item #5 in the picture below)

See this vane cleaning trick that some have used using oven cleaner to keep from having to remove the turbo.
VNT turbo stuck vane cleaning FYI

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

Greatly appreciate the help, friend.

As per usual, after fixing the problem I feel like a dummy. I ruled out a vacuum leak because I still had SOME boost (as evidenced by the turbo lag) and I had no codes for the vacuum solenoids. I pulled the vacuum line off of the turbo and blew into it, and surprise, it whistled!

It was the on/off solenoid. After bypassing it, there are no more problems. Am I safe to take my road trip with that solenoid bypassed, or are there greater consequences to leaving it bypassed?

This is all making me think the underboost condition may perhaps have always been present, and the only reason the code wasn't being thrown is because the MAP sensor was too caked in carbon to get a reliable enough reading to throw the code. This would make sense as I have been getting 17-19MPG since buying the Jeep in January (fuel economy has gradually risen with the ambient temp) and I was blaming the low fuel economy on the thermostat. However if it is actually an issue of underboost, that is a different story altogether.

Here's to hoping that fixing the boost + new t-stat will improve the MPGs. No time like a road trip to find out, I guess?

WWDiesel · **Posted:** Wed May 24, 2023 11:36 am

Very glad you found your vacuum leak and it solved your issue! :BANANA:

Problems with the vacuum solenoid is quite common and anytime low boost problems pops up, it is best to "Bypass" the solenoid first thing to rule it out.
Very easy test!

When you get time you should still consider replacing ALL vacuum lines/tubing with good high quality silicone vacuum hose since they are impervious to cracking, heat, and oil. See picture 2.

Part No. for new vacuum solenoid: Vacuum Solenoid - 4606226AC IDParts.com has them available and they are not that expensive.
https://www.idparts.com/vacuum-solenoid ... -3211.html

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

Will do. Do you have a preferred source for the silicone vacuum lines?

Also, just to confirm - I'm not going to do any damage by taking the trip with the solenoid bypassed, correct?

WWDiesel · **Posted:** Wed May 24, 2023 4:53 pm

atlantis737 wrote:

Will do. Do you have a preferred source for the silicone vacuum lines?
Also, just to confirm - I'm not going to do any damage by taking the trip with the solenoid bypassed, correct?

The only purpose of the solenoid is to maintain vacuum on the little small reservoir when the engine is shut down and the ign. key is turned off.
You should be OK on your trip, but you still need replace the solenoid ASAP.
Just let the engine idle for several minutes when you start it to allow the vacuum to build up in the reservoir.

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

Okay. Back from the trip. Less than ideal results.

The code came back after any time I had to go up a hill in 5th gear. Also there is now an "anti-sweet spot" when going uphill where the engine starts lugging and dumping black smoke, but the trans won't downshift without more pressure on the pedal. The spot seems to be around 1600-1800 RPM. At or above 1900 RPM on the same hills it seemed to have enough boost. Below that RPM on the same hills it was happy to downshift. I'm not sure if this is normal behavior or if it is related to the low boost.

I have double-checked and confirmed there is no new vacuum leak by pressurizing the hoses.

Here's the live data I could get from my handheld code reader at a steady 70MPH indicated and an ambient temp of 83F:

- RPM 1985
- TPS 9.4%
- Load 45.5%
- MAP 42.8 in hg
- Coolant 181F (this is with a brand new euro spec thermostat, and I am concerned about it)

Not sure if that data helps at all.

I guess the next thing I have to start digging into is the turbo itself. Very frustrating because the guy I bought it from said he replaced it 8 months ago and he showed me the invoice when he bought the new (not rebuilt, according to the invoice) turbo. I guess that doesn't necessarily mean he actually installed it, though. But the turbo looks pretty fresh compared to the rest of the engine.

The only other thing I can think of is either I need to gut the FCV butterfly and/or the intake might have carbon deposits. I've seen pictures of CRD intakes with some pretty crazy deposits.

WWDiesel · **Posted:** Fri Jun 02, 2023 2:23 pm

Boost pressure readings while you are getting the black smoke and low power would be very beneficial in diagnosing problem.

Yes for sure remove the butterfly plate out of the FCV, they are notorious for stripping the little small gears in it's operator and hanging the butterfly partly shut preventing full air flow/boost into the engine especially under higher load conditions.
Next, blank off the EGR exhaust feed pipe at the EGR valve. EGR valves are known for carboning up and hanging partly open thus allowing boost pressure to backfeed and be dumped off the intake manifold backwards into the exhaust manifold. (reverse flow)
You can make the EGR blank out of any thin sheet metal, even a lid from a soup or vegetable can. Tin snips and a 5/16" or 8mm drill bit is all that is needed to make the blank. See enclosed drawing below for blanks dimensions. The blank can be installed in about 15-20 minutes with simple hand tools. see picture below.

EGR BLANKS:

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

I'm back in southern Michigan, the hills here aren't steep enough or long enough to replicate the issue. But, if I can think of a way to try and simulate the condition, I will give it a shot and try to get the readings.

I will try checking over the turbo vane actuator rod and see if I can see anything that doesn't look/feel right. If it seems okay then I'll do the FCV/EGR mod and go from there.

I don't need a tune for the FCV/EGR mod, right? It's just going to throw a CEL, but otherwise it'll be fine?

WWDiesel · **Posted:** Fri Jun 02, 2023 10:58 pm

First thing to eliminate is the FCV sticking and EGR valve leakby!
Nothing wrong with checking the turbo vane movement during the process.

Blanking off the EGR valve and removing the butterfly out of the FCV may set an occasional P0101 but will not cause any limp modes or other problems. I ran with just the blank installed and the butterfly removed from the FCV for well over a year before removing ALL the EGR crap.

If you eliminate the EGR by either blanking it off or totally removing all the worthless crap, Yeti (Marco) on this forum can provide you with a custom tune that will permanently disable all EGR associated DTC's. :wink:

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

Well, I've removed the FCV butterfly but it was wide open when I did so and the gears were not stripped. It was quite springy and snapped back open with ease any time I rotated it by hand. I was really hoping it would be my issue, but alas. I scraped a lot of gunk out of the FCV pipe but it wasn't nearly enough to be part of my problem.

EGR blanking plate was made from a chopped up beer can. Took me a lot longer than it should've but all the detailed disassembly videos on youtube have been taken down so I was flying a bit blind. I at least knew to remove the fuel filter head. The hose clamp underneath the FCV pipe was annoying.

At some point in the near future I'll buy the kit to completely rip out the EGR system and I've spoken to Yeti about a tune, just have to get her to a basic level of normal function first.

I'm about to head back out and put it all back together, hope I don't run out of daylight first. I'll be able to report back tonight or tomorrow with whether or not my code comes back.

It seems nobody actually has the on/off solenoid in-stock. IDParts is out of stock, Summit is out of stock, Jegs is out of stock. The guy I spoke to at Jegs says the part has been discontinued entirely but I have a hard time believing that since it appears Sprinter vans use the part too. Any ideas before I drop $125 on the only current US-based ebay listing?

Edit: so I got it all back together this afternoon and went for a drive. When I initially started it up, it threw an overboost code. I took that as a good sign, I figure when it last shut down it was self-adjusted for the low boost so now when I started it and it got normal boost it read that as too much. Drove about 1.5 miles to the fuel station. By that time it was already throwing a P0401 for the EGR flow being blocked. I take that as another good sign that the block is working (I was worried the beer can aluminum might not be thick enough). On above-average acceleration it does still have black smoke coming out the tailpipe (no change from before blocking the EGR) but I'm not sure if that is normal or not. Coming back home I gave it WOT getting on the interstate to try forcing an underboost condition if it still existed and the CAC-FCV hose popped off so of course by the time I got home 3/4 mile later I had another underboost code and no way to know if it was there before the hose popped off. I'll just let the engine cool off and then I'll go back out, make sure it's on better, and then tomorrow morning I've got a 2 hour round trip so I'll find out then whether or not I still have a boost issue. Fingers crossed.

atlantis737 · **Joined:** Sat Jan 07, 2023 12:28 pm **Posts:** 14

Well, I THINK this might finally be resolved. Thanks for all your help.

Drove 101.2 miles including approximately 18-20 miles in the city, the rest highway @ 85 MPH (I don't normally drive that fast but I wanted to induce as much strain as I could to try and trip the code) including 8 times getting on the interstate at or close to full throttle. I do not have any underboost code. Last night I had a P0401 code for the EGR flow being blocked, then I disconnected the MAF and that code went away, now I only have the P0102 MAF code. I filled up after getting off the interstate for the final time this morning and she took 4.893 gallons of diesel for an average of 20.7 MPG. That is a significant increase, previously I was getting in the high 18s-low 19s. So, for the moment, I'm going to tentatively call this fixed. I guess the EGR valve must have been stuck open and I was leaking boost despite having the MAF closed? I need to do the glow plugs before winter comes back so my plan for right now is to do a full EGR delete and install a tune from Yeti at whatever point I do those plugs, since I have to tear it all apart at that point anyway.

I do still experience some black smoke if I quickly give it a lot of throttle at low speeds. I know this is normal on bigger diesels, I assume it is still normal on our engines?

Two questions remain:

- How ASAP do I need to replace the on-off solenoid? IDParts says they will be back in stock in July. Or I could drop $125 on the one single US-based version on ebay and have it replaced this week. I'd rather not spend double if it's not a critical fix, but I will if I have to.

- Should I keep the MAF unplugged or should I plug it back in and just ride with the P0401?

WWDiesel · **Posted:** Thu Jun 08, 2023 3:18 pm

If you have the EGR blanked off and the butterfly plate out of the FCV, you can plug everything back in or leave the MAF unplugged. It makes no difference to the engine either way. IF you plug everything back in with the above items disabled, you may get an occasional P0101. But it will not cause any problems to engine running like a limp mode etc...
The purpose of the vacuum solenoid is to hold vacuum in store for turbo vane operation when you first start the engine. With it bypassed, just let the engine idle for a few minutes to build up some vacuum for proper vane operation. :wink:

Dreaded Underboost Code

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