Need help DTC P0278

Need some help guys. 1999 GMC Suburban 2500 6.5L 140K miles has developed a vibration at idle that seems to go away when driving. Almost feels like the engine is imbalanced and it shakes the entire vehicle. Took it my GM dealer and they could not diag. Says vehicle had code P0278 Cylinder 6 Balance System code. Injection pump is less than a year old and injectors have less than 30,000 miles on them. Did compression test and all cylinders were 380+ Swapped injectors to see if made any difference, no change. No smoke or loss of power. Any help as to what could be the issue?

Welcome aboard!

Swapping the injectors is a very effective diagnostic step. If the problem stays with the cylinder, it eliminates an injector issue. Still, you need to verify the computer is diagnosing itself correctly. If it is, in fact, an issue with #6, loosen that injector line nut while idling. Note any changes. Do the same for all the others, and note any changes. Unfortunately, #6 is the least fun to mess with. If you had a shop do all the work, the shop, even a dealership, may be suspect. Unless you witnessed it, and/or have absolute trust in that shop, know that many of them take a chance and completely skip that cylinder (injector or glow plug) to save 1+ hour of extra work. There are methods to do it much more quickly, but not by the book.

Poor idle only, on only one indicated cylinder, can suggest a collapsed lifter (lazy at idle, but pumps up at higher RPM and/or oil pressure). Not a common issue, but it happens. It could also be poor or contaminated fuel, if it seemed to happen out of the blue. Try to trace back to when it first happened.

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Originally Posted by bradly202

Need some help guys. 1999 GMC Suburban 2500 6.5L 140K miles has developed a vibration at idle that seems to go away when driving. Almost feels like the engine is imbalanced and it shakes the entire vehicle. Took it my GM dealer and they could not diag. Says vehicle had code P0278 Cylinder 6 Balance System code. Injection pump is less than a year old and injectors have less than 30,000 miles on them. Did compression test and all cylinders were 380+ Swapped injectors to see if made any difference, no change. No smoke or loss of power. Any help as to what could be the issue?

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What were the readings of each cylinder from the compression test. 380 psi is low for a 6.5 Diesel. They should be over 400 psi. If 380 is the low number and the rest are significantly higher, then you have a problem on one cylinder -- the low one. If They are all under 400, the engine is very tired or the tester is not reading accurately. The P0278 code is not cylinder specific. It simply is looking at the injector fuel balance and checking the amount of fuel increase or reduction to each cylinder required to maintain smooth idle. If any one cylinder requires too much fuel increase or reduction to maintain smooth idle, the code will set. To determine which cylinder is the culprit, you must use the Tech 2 to do a cylinder balance test where you shut off individual injectors one at a time and see if the idle changes. Cracking the B-nut at the injector as DmaxMaverick suggested can substitute for the Tech 2 balance test if one is not available. Codes P301 through P308 are individual cylinder misfire codes.

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Originally Posted by ronniejoe

What were the readings of each cylinder from the compression test. 380 psi is low for a 6.5 Diesel. They should be over 400 psi. If 380 is the low number and the rest are significantly higher, then you have a problem on one cylinder -- the low one. If They are all under 400, the engine is very tired or the tester is not reading accurately. The P0278 code is not cylinder specific. It simply is looking at the injector fuel balance and checking the amount of fuel increase or reduction to each cylinder required to maintain smooth idle. If any one cylinder requires too much fuel increase or reduction to maintain smooth idle, the code will set. To determine which cylinder is the culprit, you must use the Tech 2 to do a cylinder balance test where you shut off individual injectors one at a time and see if the idle changes. Cracking the B-nut at the injector as DmaxMaverick suggested can substitute for the Tech 2 balance test if one is not available. Codes P301 through P308 are individual cylinder misfire codes.

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Technician performed balance test using Tech 2. He said the idle changed when cutting off cylinder 6, but suggested it did not change the idle as much as deactivating some of the other cylinders. As far as the compression test he indicated that the dealership purchased a special tool to perform. Tested the cylinders that were accessible, but indicated that he was not able to test compression on 6 because it would require removing the turbo. With that being said the compression was even among all cylinders that were tested. I have no knowledge of these engines, and it has performed very well for the 5 years I have owned it. What should be the next step in diagnosis?

The technician is blowing smoke about #6 being inaccessible for a compression test (he might have low compression himself). The compression test is done using the glow plug hole, which is easily accessed through the fender well. #8 can be a bear to get to on some trucks and may require removal of the inner fender for access. That is not very hard to do.

It is concerning that he claims to have swapped injectors on #6. That requires the turbo to be removed and he seems averse to doing that. I am skeptical that this dealership technician is being straight with you.

As I said before, 380 psi for a compression test is very low for the 6.5. Your original post stated that the vibration is bad at idle but seems to go away at higher engine speeds. You also said that there is no smoke. That does not fit with a cylinder miss for low compression. If the fuel system is functioning properly and you have a miss due to low compression, there will be lots of blue smoke, which is raw, unburned fuel. A miss with no smoke usually means that fuel is not being delivered to that cylinder from a bad injector, leaking high pressure line, bad injection pump, or possibly a bad PCM.

We need to confirm everything that you have been told by the mechanic previously. If you can post a video of the engine running at idle and then at higher speeds, that might help to sort this out.

I agree with RJ. Something is amiss, outside of the engine compartment. If you have a statement saying #6 was swapped, but they didn't do a compression test on that cylinder because they didn't want to remove the turbo, I'd suspect anything they say. As RJ said, accessing the #6 GP hole for a compression test isn't difficult.

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Originally Posted by DmaxMaverick

....Unfortunately, #6 is the least fun to mess with. If you had a shop do all the work, the shop, even a dealership, may be suspect. Unless you witnessed it, and/or have absolute trust in that shop, know that many of them take a chance and completely skip that cylinder (injector or glow plug) to save 1+ hour of extra work. There are methods to do it much more quickly, but not by the book....

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