Fouled O2 Sensor?
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Elder User
Joined: Dec 2016
Posts: 854
Likes: 39
From: Mid Missouri
Yes, I had no issues with the 302. I used the computer, wiring harness, sensors and all accessories from the 302 over to the 351. The air pump has been deleted but TAB/TAD is still electrically connected. EGR is functional.
It does have the 19lb orange injectors, but with a new rebuilt set I bought on eBay. The 351 fuel rail had a little crap in it, I blew it out with compressed air but still had some debris in it and clogged the #8 injector causing a dead cylinder. I replaced it with an old known working injector from the 302.
The 351 is a roller motor from a '96 F-250. Cylinder walls were not bored but honed. New rings and all new bearings used after getting correct sizes from machine shop. The pistons and rods were reused, crank was polished. Stock cam, 1.7 rockers, GT40P heads (remanufactured purchased off Rock Auto) long tube headers replaced the stock manifolds along with new catalytic converter and large body muffler, and new Edelbrock 351 Truck intake. It's not leaning out at higher RPM's.
It does have the 19lb orange injectors, but with a new rebuilt set I bought on eBay. The 351 fuel rail had a little crap in it, I blew it out with compressed air but still had some debris in it and clogged the #8 injector causing a dead cylinder. I replaced it with an old known working injector from the 302.
The 351 is a roller motor from a '96 F-250. Cylinder walls were not bored but honed. New rings and all new bearings used after getting correct sizes from machine shop. The pistons and rods were reused, crank was polished. Stock cam, 1.7 rockers, GT40P heads (remanufactured purchased off Rock Auto) long tube headers replaced the stock manifolds along with new catalytic converter and large body muffler, and new Edelbrock 351 Truck intake. It's not leaning out at higher RPM's.
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Elder User
Joined: Dec 2016
Posts: 854
Likes: 39
From: Mid Missouri
So you think those are fuel fouled plugs? The threads on the plugs that had 2k miles on them had oil on them, although the ones I just replaced 100 miles ago do not, but are fouling. The tips of the plugs are not wet, just a black soot-like coating.
I changed the oil a few days ago and did not smell gas in it. I get a small cloud of smoke on a cold start-up, it appears to be blue and cannot tell if smoke blows out on a heavy acceleration.
I have not tried disconnecting the MAF sensor. I did clean it, however, it didn't look dirty to begin with. I'll disconnect it and see what happens.
The exhaust doesn't smell rich now, it was way rich when the pushrods that were too long were still in. As soon as I put shorter pushrods in, the extreme rich smell went away. Although the tailpipe is fouled out too and the truck gets poor gas mileage (about half what it was when the 302 was still in).
I changed the oil a few days ago and did not smell gas in it. I get a small cloud of smoke on a cold start-up, it appears to be blue and cannot tell if smoke blows out on a heavy acceleration.
I have not tried disconnecting the MAF sensor. I did clean it, however, it didn't look dirty to begin with. I'll disconnect it and see what happens.
The exhaust doesn't smell rich now, it was way rich when the pushrods that were too long were still in. As soon as I put shorter pushrods in, the extreme rich smell went away. Although the tailpipe is fouled out too and the truck gets poor gas mileage (about half what it was when the 302 was still in).
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Elder User
Joined: Dec 2016
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From: Mid Missouri
Oh, and it's consuming a lot of oil, about a quart every few hundred miles. It has a minor leak, coming from either the rear main seal or pan gasket.
If the pushrods are still too long, would that cause oil to get into the combustion chamber or run rich causing this issue? But again, vacuum readings are perfect. The thing is, when this engine was still on the stand I measured for correct pushrod length, or so I thought. Once engine was in and running, the pushrods had to be swapped out for shorter ones.
Compression is good and even throughout the cylinders. I still need to test two cylinders where spark plugs cannot be removed because of the long tube headers. A special spark plug socket is on order.
Thanks for all your help.
If the pushrods are still too long, would that cause oil to get into the combustion chamber or run rich causing this issue? But again, vacuum readings are perfect. The thing is, when this engine was still on the stand I measured for correct pushrod length, or so I thought. Once engine was in and running, the pushrods had to be swapped out for shorter ones.
Compression is good and even throughout the cylinders. I still need to test two cylinders where spark plugs cannot be removed because of the long tube headers. A special spark plug socket is on order.
Thanks for all your help.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
OK that does sound more like oil fouling then. If that is the case then what is causing it? Are the rings not seated? Is it pushing a lot of oil up into the intake through the PCV? Or is it just running rich because the O2 sensor is fouled and the extra fuel is washing oil off the cylinder walls.
What is the fuel milage?
What is the fuel milage?
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Elder User
Joined: Dec 2016
Posts: 854
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From: Mid Missouri
Right, it could be rings not seated. Engine now has 2,500 miles so I would think they are seated. I "broke it in" for first 500 miles, varying engine speed. I'm waiting for the spark plug socket so I can do a leak down test.
I've looked into the PCV system, the valve is new and is working as it pulls a lot of vacuum if it's removed. How can I test the PCV further to see if its sucking oil through the intake?
Would the fouled O2 sensor throw a code? As stated earlier, I'm not getting any codes. My guess is the O2 sensor got fouled after driving with the exhaust leak for first 2k miles or so?
I haven't calculated fuel mileage since the 351 swap, but it's definitely worse than when the 302 was in, which averaged 14 MPG. I'm driving on the same streets and same conditions as before.
The kicker is, this motor seems to run like a champ. It doesn't hesitate, miss, sputter, back fire, stall, or fall on it's face. It starts up every morning but something cannot be right with the way it's consuming so much oil and quickly fouling plugs.
I've looked into the PCV system, the valve is new and is working as it pulls a lot of vacuum if it's removed. How can I test the PCV further to see if its sucking oil through the intake?
Would the fouled O2 sensor throw a code? As stated earlier, I'm not getting any codes. My guess is the O2 sensor got fouled after driving with the exhaust leak for first 2k miles or so?
I haven't calculated fuel mileage since the 351 swap, but it's definitely worse than when the 302 was in, which averaged 14 MPG. I'm driving on the same streets and same conditions as before.
The kicker is, this motor seems to run like a champ. It doesn't hesitate, miss, sputter, back fire, stall, or fall on it's face. It starts up every morning but something cannot be right with the way it's consuming so much oil and quickly fouling plugs.
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From: Long Beach, WA
The cam is the factory F4TE roller cam. I did however, install 1.7 rockers. There was an issue with the pushrods being too long-the valves were sticking open. I installed the factory length pushrods with the exception of one, which was too short. Before realizing the pushrods were too long, the engine was running very rich, and I had to advance the timing to almost 40 degrees BTDC just to get it running. Timing is now set to 12 degrees BTDC. I initially thought the fouled plugs were from when it was running real rough but now that that issue has been cleared up, I'm still getting fouled plugs. Perhaps the pushrods are still not quite right and is allowing oil into the combustion chamber somehow.
Are the engine vacuum readings you posted current? The engine vacuum readings were very good for an engine that is running rich. I would expect more like 17 in.hg max unless there is some advance dialed in to create a little more vacuum. I worked at a dealership in Prescott AZ, which is the "other" mile high city and 18 in.hg was very good. The needle was steady at idle?
, not bouncing 5-7 ?Have you taken the plugs out and cleaned them or replaced them? Fouled plugs will not usually clean themselves.
I'm a little puzzled about the pushrod length situation. Before I would pull the heads I would put the factory push rods and rockers back in and see how it runs...
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Elder User
Joined: Dec 2016
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From: Mid Missouri
F250Flyer,
Yes, this is a rebuilt 351 replacing the 302. The engine vacuum readings I posted is current, around 18 in. at idle. I rebuilt this engine in Missouri closer to sea level and vacuum was 21 in. there. The needle is steady at idle, just a very slight movement. The vacuum readings are picture perfect to the first two examples of a "Normal" motor found here: TRS Magazine - Dealing with engine vacuum
I replaced the new plugs that were put in for first start up a couple weeks ago with another new set, which are already fouling. I'm now on my 2nd set of plugs with only 2,500 miles on engine. Both sets are Autolite coppers for a late 90's Explorer with GT40P heads.
I like the idea of putting the stock rockers back on before tearing heads off and seeing how it runs.
Perhaps these plugs are fuel fouled and oil fouled? I don't know if the fouled O2 sensor is a result of the engine issues or if the O2 sensor is to blame. I am stumped at this point and appreciate all the help and ideas I can get!
Yes, this is a rebuilt 351 replacing the 302. The engine vacuum readings I posted is current, around 18 in. at idle. I rebuilt this engine in Missouri closer to sea level and vacuum was 21 in. there. The needle is steady at idle, just a very slight movement. The vacuum readings are picture perfect to the first two examples of a "Normal" motor found here: TRS Magazine - Dealing with engine vacuum
I replaced the new plugs that were put in for first start up a couple weeks ago with another new set, which are already fouling. I'm now on my 2nd set of plugs with only 2,500 miles on engine. Both sets are Autolite coppers for a late 90's Explorer with GT40P heads.
I like the idea of putting the stock rockers back on before tearing heads off and seeing how it runs.
Perhaps these plugs are fuel fouled and oil fouled? I don't know if the fouled O2 sensor is a result of the engine issues or if the O2 sensor is to blame. I am stumped at this point and appreciate all the help and ideas I can get!
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
The idea with disconnecting the MAF meter is to see if the computer is in limp mode or not, so you want to do this on a warmed up motor. If disconnecting it makes no difference at all to the way the engine runs then you now know the computer is ignoring it, and that means something is forcing the computer into this state and your problem is EFI system related. But if disconnecting it does make a difference then you know the computer is trying to control the motor normally so your problem is not EFI related.. more likely a mechanical problem.
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Elder User
Joined: Dec 2016
Posts: 854
Likes: 39
From: Mid Missouri
No, I'm using stock valve covers and did not do anything to to the baffling, the 1.7 rockers cleared no issue. I'm going to look through the throttle body plates to look for oil splatter on the upper manifold. Maybe you and Conanski are on to something with it being PCV related.
Cargo Master
Joined: Mar 2010
Posts: 3,330
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From: Laramie, WY
I'd start with the MAF meter unplugged test.
I just wanted to make sure you had the baffles in place. If you pull the oil fill cap while it's running, is there vacuum there, at the valve cover?
I just wanted to make sure you had the baffles in place. If you pull the oil fill cap while it's running, is there vacuum there, at the valve cover?