How to Test the Upstream Oxygen Sensor (2003-2005 GM 4.8L, 5.3L, 6.0L)

I love that the Internet allows us to learn from other's experiences. I have learned a lot myself over the years and this website is my way of giving back. The post in this page is one that offers several important lessons...

Although this the information in this article pertains to a GM 3.8 liter V6, it can be applied to anything rolling around on pavement that has a water-cooled engine.

NOTE: This tutorial supplements the information found in this article: How to Test a Blown Head Gasket (GM 3.8L).

... and with Dave's permission, I have posted it in its entirety:

Initial Post: Water Pump Failed and Misfire Codes P0303 and P0304

This is the first email I received from Dave explaining that his water pump failed and asking some more specifics about the compression test he had performed:

“I lost a water pump going up a 6% grade incline at high speed and wasn't watching the temp. gauge.”

“... I used some Duraseal to overcome some coolant leak problems, but still had a 70 lb. compression reading on #3 and #4 and was throwing P0303 and P0304 codes. Even after I solved the coolant leaks, which I don't know if those were from the head gasket or the Intake Manifold, I was still getting those codes.”

“... Another compression test gave the same results, and when we added a spoonful of oil (for a wet compression test) it only came up ten pounds, to about 80 psi. At that point, I figure that it may be a valve problem, a burnt seat or a bent valve, etc. I figure I need to rebuild or get a used engine installed. Any experience or light to shed would be appreciated.”

“... I notice that in your article, you don't say much about what the conclusion is if the compression test is bad, or discuss the leak down test to confirm if the problem is from the rings, the intake valve, or the outgassing valve.”

Update: Blown Head Gasket Confirmed -Solution: Replace the Engine

“Here is an update for you from my experience:”

“Based on the fact that a wet compression test only brought the value up to 80 psi, vs. the 120 in other cylinders, I came to the conclusion that I had a valve problem due to overheating. (This might be something to add to your article on analyzing the compression test.)”

“The DuraSeal headgasket treatment I used (without any sodium silicate in it’s formula) did seal the head gasket problem, but made no difference on the compression values, obviously). I wanted to believe that I could solve the problem without tearing into the heads, but the reality is that even if the Intake Manifold Gasket were bad, or if the plastic Intake Manifold were warped (as the local Buick Dealer Tech suggested), that would not change the compression problem. The initial compression test indicated a problem and I wanted to believe a head gasket problem was the source of that loss of compression. But even after doing the chemical fix – which worked – I still had loss of compression. With the wet test showing little improvement, the inescapable conclusion was that the valves were compromised. I didn’t do the leak down test to confirm if it was the intake valves or the exhaust valves. All I knew is that the valves were bad.”

“At that point I looked at the cost of paying someone to tear it all down, then magnaflux the heads and fix the bad valves and put it all back together: not a small amount. The problem is that even once you do that, if you do that, you then have a strong upper engine working on a 170,000 lower engine. The prospect is that then – as many times it has been seen – that later the lower end is blown out and you have to start all over again to do a ring job. Tilt!”

“This is what I did:”

“I called the S** V**** area salvage yards to find a low mileage used engine in the L*. area. I was referred to one that had some model year 2007 Pontiac Grand Am 3800 engines that came from some rental cars that were totaled. I looked at two engines that looked clean and given the VIN on each. One proved, with a CarFax, to come from 2007 Grand Am that had mileage of only 26,541. Eureka! The Buick dealer verified that although the engine parts were not exact, the only difference appeared to be the type of multiport injections systems employed. The salvage yard gave me a price of $1,500 to buy the engine AND install it. He used my existing plastic Intake Manifold, not the metal one on the ‘new’ engine, and got it to work. No codes, other than a P0404 on the EGR valve, which may need replacing. But the engine runs like a top, and for a price that is less than or on a par with the cost of removing the heads and having them done. And I have a 90-day parts and labor warrantee. The car now has an engine with about 27,000 miles on it and a rebuilt tranny from last April. That should have a lot of life left in it.”

“I asked them to pull the heads off my old engine – which was really dirty – so I could confirm my diagnosis. I found the exhaust valves to be burned. So it confirmed that I made the right call. With that kind of mileage, why just fix the top end? The cost of a total rebuild or of a ‘Crate Engine’ was beyond my budget. ($1,700 to $2,300 for the engine and then another cost to remove and replace -- or install -- on top of that).”

The Real, Bottom Line ‘Mechanical’ Life Lesson I Learned...

“The real, bottom line ‘mechanical’ life lesson I learned? Do you want to know?”

“My initial damage came from going up a long 6% grade and not paying attention to the temperature gage to see the rise in engine temperature from the failure of the water pump. The engine may have gone a long time if I had replaced the water pump before it failed. For several months I had noticed the coolant level going down an inch or so over the course of a few weeks and then repeatedly topping off my reservoir. Although I had taken the car to my mechanic and he was aware of this coolant loss, NO ONE EVER SUGGESTED WE EXAMINE THE WEEP HOLE ON THE WATER PUMP! (what kind of mechanic is that?)”

“That is the key thing I learned. IF YOU ARE LOSING COOLANT OVER TIME, CHECK THE WEEP HOLE ON YOUR WATER PUMP FIRST. That is the first place to look. If I had done that and replaced the water pump when it was telling me it was about to go, I would have avoided all the time and cost of the repairs I incurred. But now I am even better off, in a way, with a low mileage engine to go with my already rebuilt tranny.”

“I know of another man who had a similar experience: he was losing coolant over time and then blew the water pump which caused a blown head gasket. They started with that and got into the heads and then a valve job. By the time he was done, the bill went to $3,000 or $3,500 and he had to walk away from the vehicle and surrender the pink – he couldn’t afford the ride his mechanic took him on. AGAIN, no one told him about checking the water pump when he was losing coolant. This could be ignorance at best and it could be intentional negligence at worse – but it is reprehensible.”

“When I took the old water pump out, it was OBVIOUS that I was losing coolant. But the weep holes are not that easy for the average person to inspect without a mirror on a stick and the KNOWLEDGE of what to look for.”

“I take the time to tell you all of this so that you, in your position, and with your platform, might pass the message on to help some others avoid the expense of a lost engine when a simple water pump replacement in a timely fashion is all that is needed. An engine’s greatest enemy is excessive heat. The water pump is a key component to that protection. Most people don’t really appreciate this or know what to look for or what to do. I exhort you to help them!”

Kind regards,