LHM limp home mode possible causes

Aqua Puttana said:

Assuming that the turbo air delivery system design is a bit over-sized overall, the amount of restriction provided may have gone unnoticed for some time.

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sikwan said:

You bring up a good point and there's only one way to find out and that is to leave the P2359 code with and without the sleeve impeding flow.

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surlyoldbill said:

I Asked years ago about GDE working to replace a failed EGR by tricking the sensor, and no one could confirm.

Anyway, what I learned about 02-03 EGR:
-They fail eventually and are a maintenance item, you'll need to replace them every 100-200k (depending on driving habits),
-They fail via the seal around the shaft between the cam chamber (under the plastic cap) and the main intake. It wears and allows too much boost pressure to escape, and the sensor readings of MAF and Boost pressure do not jibe, so the ECU thinks there is a boost leak (there is) and goes into LHM.
-They cannot be repaired as far as I know. Everyone THINKS it's the valve itself, between the exhaust and the intake, but that's not the problem, and cleaning that to look like new won't "fix" the problem. It might prolong lifespan of the EGR because it may reduce sideways pressure on the valve arm, so it doesn't wear the seals as fast. It sure doesn't hurt. Using a block-off plate over the exhaust inlet will not "fix" the LHM issue, confirming that it is not the problem with a failed EGR.
-Wahler is the one and only manufacturer, so don't worry about getting a "cheap Chinese part".
-dripping oil from the cam chamber is normal, and is NOT a sign of a failed or failing EGR. Actually, they are failing from the first mile driven. However, the more the seal gets worn, the more oil vapor gets through, and the more they will drip.
-There is no reliable advanced warning that the EGR will send you in to LHM. It will do it sporadically and a re-start will "fix" it, but these instances will occur with more frequency over the next few months until you cannot get it to stay "fixed" by restarting. That first couple LHM episodes is your warning, and you should order an EGR.
-It takes less than 10 minutes to replace the EGR, and no special tools.


*often people think their turbo is failing because they lose boost, or that there is a leak in the charge air system, or maybe a failed turbo actuator, or disconnected vacuum line. Both times when this happened to me I went down the turbo and turbo controls rabbit hole, and both times it was the EGR. I believe I replaced the OEM EGR at 150k, and the next one at 250K.

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Nautamaran said:

I agree with Dick's explanations, though he left out the O2 sensor feeding back a Lambda “excess oxygen ratio” value, which accounts for much less fuel injected than the air passing through the MAF sensor would indicate. We can’t allow all the O2 to be consumed during ignition or there isn’t any left for the catalytic converter to work with, but if we let ALL the air in at low load we get too many NOx compounds, so EGR is increased to displace some of the fresh air that would otherwise enter the cylinders.

The ECM supports two “generic” SAE Mode 1 pids for air pressure:
PID 0B - Intake Manifold Absolute Pressure (kPa)
PID 33 - Absolute Barometric Pressure (kPa)

Your generic scan tool or app may do the math and display "Boost" = MAP - Barometric.
A T1N specific tool like the Autel MD802 or AP200 can decode the Mode 21 data block that contains all three absolute pressure sensor values.

If Intake Pressure (sensed in the filter box on the clean side of the filter) is much below ambient Barometric pressure (sensed under the dash by the ECM) then there is a restriction in the air intake, likely a clogged filter, but could be a plugged fender grate or hose.

If your MAP (sensed on the hose rising from the cold side of the intercooler) is near the Barometric Pressure then the turbo is idle.

With around 14 psi ambient, I will occasionally (though rarely) see 35 psi MAP, or 21 psi "Boost". A more typical MAP value for my driving style is 18-24 psi, or 4-10 psi of boost.

An off-thread (but related) adventure tale:
I got to watch my Arduino display as the ECM coped with a split induction hose today... we felt a subtle saw-toothed surge on our way out of town yesterday, but things looked okay. On our way home this afternoon, it was much more pronounced as the engine hunted for a combination of turbo actuator (85%-60% = 15% to 40% vane engagement), MAP (18 to 24 psi), and likely an EGR and fuel quantity that would maintain the cruise control's set speed of 65 mph. I dropped to 60mph, but the symptoms continued and it finally gave up and went into a No-Turbo limp mode. It could achieve 60 mph down hill, but with just 125 ft-lbs coming into the torque converter the van slowed to 40 mph on the slightest uphill grade. A key-off / key-on reset helped, but LHM would return at the next hill, even driving without cruise and holding MAP below 20 psi. I found it interesting that the LOD value re-calibrated while in limp, staying pegged at 100% with a MAF of around 150 g/sec at around 2500 rpm and 14 psi MAP. After the key reset, MAP could (briefly) reach 24 psi (10 psi "boost") and the MAF would climb to about 190 g/sec, with a LOD of 65%. I didn't note the EGR commanded... and am wishing I had asked my copilot daughter to take a video of the Arduino screen (or perhaps had calbiker's logger sketch loaded?)

At the next town I found an auto parts store, and used my MD-802 (don't leave home without it!) to scan the ECM before shutting off the engine: "Pending/Stored" codes 2359-1 (Charge Pressure Control - charge pressure too low) and 2359-2 (Charge Pressure Control - charge pressure too high). Not unexpected. I shut it off and popped the hood, expecting maybe a loose clamp or an obscure hose tear somewhere, but was instead presented with a 2" tear front and center.
Good karma? I'll certainly take an easy diagnosis over a head-scratcher any day, or a crawl under the radiator checking hoses.

So a roll of silicone self-stick tape patched the hose, cleared the DTCs from the ECM, and we were on our way... remaining two hours home without incident; no codes pending.

The patch will likely hold for a while, but I'll order a set of induction hoses first thing in the morning.



-dave

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