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2001 Sonata - P0171, Surging, strong exhaust smell

Its throwing P0171
At cold start, stalls unless throttled up continually. Very rough idle when warm, violently surges at acceleration.
Strong almost sweet exhaust smell. Black soot shoots out of exhaust.
Quite significant back pressure coming from oil fill opening, popping out dipstick.
No significant vacuume leaks via smoke test
New MAF sensor
New Knock sensor
Possible new O2 sensors
New Fuel filter
Cleaned EGR valve
Cleaned PCV
Cleaned fuel injectors
Operational air filter
Operational TPS
Im wondering if I’m dealing with a blocked exhaust.
Any ideas?

Does it smell like coolant? Maybe you have a cracked head or block and combustion chamber pressure is getting into the crankcase. The sweet smell would be coolant getting into the combustion chambers and burning.

You can test for that with something like this:
https://www.amazon.com/Lisle-75500-Combustion-Leak-Detector/dp/B0007ZDRUI/

Black shoot coming out of the exhaust is a sign your engine is burning oil.Have a compression test done.

Black soot comes from incomplete combustion of the fuel (or rich metering, but I don’t think that’s the problem in this case).

Pressure to the dipstick tube comes from the crankcase. A clogged Positive Crankcase Pressure (PCV) valve can cause it to build to excess, but in this 16 year old engine I suspect it’s cylinder wear. All engines have some level of combustion that blows past the rings and pressurizes the crankcase (blowby). The weaker the seal is between the rings and the cylinders is the more the crankcase becomes pressurized from the combustion gasses. In severe wear cases, it can blow the dipstick out.

I too recommend a compression test and a “leakdown” test. You can do both while you’re there with the sparkplugs out. The first will test for excess wear, the second for conditions such as a breeched headgasket that might affect the ability of the cylinders to hold pressure. The test kit(s) are affordable, and the tests are pretty definitive.

Post back with the results.

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Assuming routine maintenance is up to date, engine and valve timing check out ok at idle, then blocked exhaust, EGR sticking open, or PCV system clogged up or hoses hooked up wrong is where I’d focus w/this problem. Start w/the EGR, that’s pretty easy to test if it is sticking open or not. It shouldn’t be open much if all at idle, so figure out a way to control it to open as a test, the engine should stall out or run much rougher at idle with the EGR open. Does it?

I would not expect stuck oil control rings to make themselves known during a compression test

I tested the compression 211psi, 210psi, 210psi, 210psi. Surprisingly strong and consistent, it does only have 97,000miles on it though. Haven’t done the leak down test.
Are we leaning towards a blocked exhaust?

I took the EGR off and cleaned it up with some carb cleaner, its diaphragm does hold vacuum. I also cleaned up the PCV with carb cleaner. The engine does have a rough idle especially when cold, stalls repeatedly unless throttle kept up until warm.Haven’t tried the “hold EGR open” test yet.

The problem with the blocked exhaust theory, the symptoms for that show up more at higher power usage, like accelerating up a freeway on ramp. Could be that, but it would have to be really blocked to cause the idle problems you are reporting. What does your intake manifold vacuum measure at idle? If 17 in hg or better, very unlikely these problems are explained by a blocked exhaust.

The compression numbers look very good. I’d defer on the leak down test. You could still have an oil control ring problem, but that wouldn’t show up in a leak down test. The surging you are experiencing could be caused by too much fuel being injected, a stuck-on injector maybe. With that problem, at idle it would run rich and not idle well, and a lot of black smoke out the exhaust comes with being too rich, and when you step on the gas more air is allowed in which, when mixed with the extra gas, will cause an engine surge. The problem w/this theory is that you are getting a lean code, not a rich code. A lean code would usually be caused by an intake leak or an exhaust leak. Let us know what the intake manifold vacuum reads, that’s something to start with. And the EGR test results. What do the plug tips look like? Black soot on them? Greasy black stuff on them? What about the end of the tailpipe, what’s that look like? You’ve taken apart & checked the entire path from the air filter to the throttle body for obvious leaks, right? And used the starter fluid spray method to find intake air leaks, right?

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It actually runs better at high power usage. When I have it warm and open it up it runs great, until I let off the gas, and that vacuum pressure tries to build…?

I did take the injectors out and cleaned them pretty thoroughly, while actuating them.

Its not so much that its surging as much as its being restricted, the surging comes from the release of that “restriction” ie big drop in RPMs

Black soot on them, looks a lot like whats being shot out of the tail pipe onto the floor.

I put a smoke machine on it. There’s a bit escaping from the throttle plate spring area, not a whole lot though; nothing from around intake manifold or along air conduit. Ive also used propane -and brake cleaner around air intake, throttle plate spring, and manifold; no bump in RPMs. My eyes were irritated from exhaust though, I assume it was coming from oil dipstick that pops out because of the strong back pressure.

Ill run the vacuum manifold and EGR tests and post back. Cheers!!

Suggest to replace the PCV valve with a new one. Inexpensive part. And if it is sticking you’ll have more problems than just black smoke out the exhaust pipe. Damaged crankshaft seals eventually, something you definitely don’t want. The dipstick popping out can be a symptom of a problematic PCV system. As I recall you have really good compression numbers, so it seems unlikely the crankcase pressure could be caused by leaky piston-compression rings.