BTC1984

2008 Chevy Suburban 5.3 liter, 155k miles. My oil sending unit was bad, so my mechanic replaced it and cleaned the filter. Immediately afterwards the engine developed a misfire in one of the cylinders. He did a compression check and the compression was not good. He said it's probably a lifter or valve related to the fuel management system. The vehicle did not have a misfire when I took it to the mechanic. The misfire started when he worked on the vehicle. Is it possible that he did something wrong to cause this problem?

oilcanhenry

He could have messed up something installing the oil
pressure sender. Did he test all the cylinders?

BTC1984

I'm not sure if he tested them all. I just know he said one of them was misfiring. I assumed he could not have "caused" the problem while doing the repair. But from what you are saying it's certainly possible. Do you think I should go back and talk to the mechanic to hash this out further? Or even to suggest he might be responsible? I don't know much about cars so I am reluctant to even suggest he played a role in this, but it is very odd the problem started immediately after he did the repair.

Irish_alley

what type of "mechanic" was this?
is it at a reputable shop or some back yard mechanic?
did it set off the check engine light? if so what code?

BTC1984

High quality mechanic with a great reputation. ( at least the owner is high quality, I don’t know which of his guys did the actual work). P0172, 175, 523, and 573. His note says possible bad afm lifter

oilcanhenry

I didn't mean to say that he did something on purpose, although there are dishonest mechanics, just like other dishonest people. He may have accidentally done something to cause this, but he should also be a good enough tech to figure it out, if he knows his business. You can have a cylinder with low compression that will still fire properly, this is usually caused by engine wear on the pistons and piston rings, but it could be a bad valve as well, or something else, like a hole in the piston(s) with is not all that common in stock motors, or perhaps a bad head gasket which can be very common on some makes of engines no matter the manufacturer or brand of vehicle.

During a dry cylinder leak down test all the spark plugs are removed, and the pressure gauge is moved to each cylinder one at a time, the engine is cranked over and the reading is written down for each cylinder. Lets say your engines normal compression is 150 PSI, but you have cylinder(s) showing 120 PSI or less. That is not within range. I read that your 5.3 Liter should be in the 160-170 PSI range in a good running engine, so no cylinder should be below 135 PSI or thereabouts.

Ideally, the cylinders will have pressure readings withing 15 percent or less variance between all the cylinders. To test the pistons,rings and cylinder walls for wear after a dry test, a wet leak down test is performed by adding a small amount of motor oil to each cylinder then cranking the motor over with the pressure gauge installed. If the readings improve, you will know that its not the valves that are an issue.

A much simpler test is using a vacuum gauge hooked up to the intake system. If it fluctuates more then a very small amount, then you have an issue, otherwise the needle will remain pretty steady. Its not a good as a cylinder pressure test, but cheaper and easier to do, and if it is not normal then a cylinder testing procedure is called for, although a bad valve can cause this too, or an intake or fuel induction system leak at the throttle body.

Now that you know more about engines, I would definitely ask his what the readings were and if he tested all the cylinders for compression readings. I would ask him why he now says a cylinder is low on pressure and tell him the motor was running fine before he replaced the oil pressure sender, which simply screws into the engine, and has
one or two wires that come out going to your oil pressure dash display. I would not accuse him of doing you dirty, since we really dont know that. If you can afford it, you might take it to a different tech and see what he says about this matter. I would not blame the tech until you know more about what he says might be the problem.

Also there is a car complaint about misfiring and excessive oil consumption: https://www.carcomplaints.com/Chevro...e/2008/engine/

What was it that the oil pressure sender that it needed changing? Do you suffer from too much oil consumption? Seems that the oil pressure sender is a common item to replace on the 5.3 liter Tahoe, when most vehicles never need it changed out. I own a 1998 Chevy K-1500 and its never been replaced. Also seem to be issues with the valve lifters and active fuel management (AFM) system as well.

KChevy75

Definitely possible as I'll outline below. There's nothing that they could have done to cause an actual collapsed lifter or damaged valve, however. Probably a lot more info below than you wanted but this will be helpful for others searching for the same thing.

Do you hear any lifter tick or knocking?

What oil do you use?

How new are the plugs and wires? Disconnecting wires from the plug that have been attached for tens of thousands of miles almost always rips the connector off the wire. If so, the wire needs to be replaced. Were the plugs purchased from a reputable source such as a local parts store or were they ebay or Amazon purchased? There are so many counterfeit plugs being sold, it's mind boggling.

Do you know which cylinder they say has low compression? If so, examine the wire and plug for damage. Check the injector plug.

Replacing the oil sending unit on the 5.3L is a somewhat big job. I've heard of people doing it by only removing a few things on the drivers side of the engine but I tried and it's pretty much impossible with my size hands.

The correct way to do it involves removing the intake manifold which involves unplugging electrical connector to things such as the injectors.

I don't know if they removed the manifold or just a few things on the driver's side but It's completely probable that something was unplugged and not plugged back in or even damaged. There could be a vacuum leak because something wasn't plugged back in.

Don't let them do any work until you perform some troubleshooting steps. Mechanics know that replacing the lifters on this engine is a huge money maker for them as it's a whole lot of labor.

First, verify that all hoses are hooked up. There's one that I always forget to hook back up. It's on the passenger side and plugs into the large plastic intake tube. On the back of the driver side there's a hose that connects to the valve cover. Make sure this is hooked up and not cracked or damaged.

Next, make sure all the electrical connectors are plugged into the ignition coils and the fuel injectors. All are easily accessible.

If they did take off the manifold, the gaskets may not be sealing properly now causing a vacuum leak. No easy way to test this. Spraying some starter spray around where it attaches to the engine might cause the RPMs to increase which would indicate a leak.

Make sure all the clamps from the air filter up to the throttle body are tightened.

If all checks out, you need a device to scan for misfires. I came across an awesome device made by Autel for WalMart under the HyperTough brand. It's the HT200 WalMart Link to HT200. This thing is like a $300+ scanner for only $50. Don't believe the reviews. This thing is just amazing. You plug it into the OBDII port then install the HT200 app. If you don't read the instructions, it's going to be a little tricky to get it activated. Read the instructions! Once activated and ready to go, you get one free download of manufacturer specific data. Go to the Mall from the app and Pick GM and download it. It's 566 MB. Once installed, you have access to way more than what generic OBDII gives you including around 20 modules in your car.

I believe you want Engine Control Module. Under there is Misfires. While idling in park, see if the count is going up for any of the cylinders. If not, press the throttle and release and see if any show up. This will generally cause misfires in small numbers even in a properly running 5.3L but if you see more than just 1 to 5 for any cylinder, keep note of which one. Next, have a partner watch the app while you drive. Have them look for multiple misfires and document the cylinder.

If any cylinder shows a misfire, pull the plug and examine it and the wire for damage. If it looks good, swap the plug for one in another cylinder, Check for misfires again. If the misfire stayed in the same cylinder, swap the wires. If the misfire stayed in the same cylinder, swap the ignition coil and check for misfire again. If it stayed in the same cylinder, remove the connector from the fuel injector and check it for damage. If the misfire moved to the other cylinder for any of the above steps, you've found the part that's causing the problem. Replace it. If still the same cylinder, you can check compression.

If you want to verify compression, you'll need a compression tester. Autozone has free loaner compression testers. They charge you up front for the tool ($40 I think) then refund your money when you return it. You have to disable the fuel injectors and / or fuel pump. There are three fuses that I remove. INJ A, INJ B and Fuel Pump. I then unplug the connectors from the ignition coils. Remove all spark plugs then starting with Cylinder 1, install the compression tester. Just hand tighten until snug. Don't use a tool to tighten. Crank the engine until the gauge stops moving. Usually about 4 cranks is enough. Record the number on the gauge. Press the button on the gauge to release the pressure. Repeat again if you want and average the numbers. Move on to cylinder 2 and so on documenting as you go. For comparison, I did a compression check yesterday on my 2007 Suburban with 290,000 miles and the numbers ranged from 175 to 195. You take your highest cylinder and multiply by .15 (15%). Your cylinder with lowest compression should be greater than this number. Anything lower then you might have an issue in that cylinder.

If compression if fine on all cylinders, I'd replace the plugs and wires if they have lots of miles on them and see if the problem goes away.

BTC1984

Just heard from the chevy dealer. I have an AFM/DOD lifter failure. They say I need a new engine. What should I do?
1. Try the fix I see on youtube where the guy hammers the lifter free then deletes the AFM system?
2. Get a replacement engine and have the dealer do the install? ($7500)
3. Go to a less expensive but reliable place like Fraser engines and have them replace the engine? ($4800)
4. Sell the Suburban as is and be done with it?
Thanks

KChevy75

Do you hear and knocking or loud ticking? I personally wouldn't take their word on it. $7500 is about double what the job typically costs including the engine.

I'd personally do a compression test first before I'd take anyone's word on something where they know you have no way of knowing if they are telling the truth. You could just have a bad plug or wires or ignition coil. A compression test is cheap, easy and there are plenty of videos on youtube on how to do it. A $40 compression tester and a typical ratchet set has everything needed to do the job.

Jim F34

Did you ever get this resolved? My mechanic recently replaced my oil sending unit on my 2013 Yukon Denali. Drove it for 40 miles now I am getting a PO300 error engine code now and hear more engine knocking than normal. Seem odd that this happened shortly after the oil pressure sending unit was replaced. Thanks,