Can an Exhaust Manifold cause High Positive Fuel Trims?

[SwampSurvivor]

I recently started to have an issue cold starting my truck on warm days. On colder days, it doesn't seem to be too big of an issue - my guess is that it takes longer to hit closed loop. It's a 2001 F-350 5.4L 5 speed. Engine has north of 120K miles on it. New intake manifold and gaskets. New left side exhaust manifold, double gasketed.

Problem: When I cold start my truck on warm days, it has a very bad multiple cylinder misfire after about 15 seconds. To the point that there's gas dripping out the muffler. It will run extremely poorly and smell like raw gas for a minute or two, then clear up and idle/run smoothly. Any restarts after that run just like a new truck. I recently took it on a trip, empty, and it got 14 MPG. That's as good as it's ever got

The passenger side exhaust manifold is very rotted out. I'm replacing that next month as time allows.

I have had people tell me it has bad valve seals causing it to foul the plugs with oil (which could be a thing, but I have no visible smoke on startup), or that it has a leaky head gasket --- I haven't had any change in coolant level. I marked it on the pressure bottle when I started having this problem and it hasn't gone down.

I plugged in the scanner and this is what I'm seeing:

Long Term fuel trim is at 3.91% for bank 1. Bank 2 long term fuel trim is at .05%.

When I start the truck and let it idle, bank 1 fuel trim will go up to 28% or so, sometimes more or slightly less. Obviously the higher it goes, the worse the truck runs. If I rev up the engine to 2500 or 3000 RPM, it will go down. to 11%-15% and it will run better and stop misfiring as horrible.

Then after 2 minutes or so of idling, the fuel trim for bank 1 will drop down to 4 or 5% and the truck will run perfect again.

From what I understand - this points to a bad vacuum leak? A bad exhaust manifold would be okay at idle and then cause positive high fuel trims when revved up , right? I've sprayed starting fluid and carb cleaner at different gaskets and vacuum junctions around the engine and I can't find any leaks. Intake gasket(s) seem to be fine as far as I can tell. As is PCV system and the lines to the evap canister. Truck doesn't have EGR, even if it did, that would be on the driver's side (bank 2)

When slowing down in gear, both banks have close to the same fuel trim. If there was some sort of severe vacuum leak on bank 1, seems like a high vacuum situation like decelerating in gear would affect it.

Help?

 

[supton]

How old is the left bank first O2 sensor? I'm wondering if the heater is lazy. It's in closed loop but maybe the O2 sensor reading is going the wrong way until it heats up.

Can you plot both first O2's and see if they track properly?

 

[mk378]

Exhaust leaks before the O2 sensor let air in, causing a false reading from the O2 sensor. The PCM will think it is running lean when it is not.

 

[SwampSurvivor]

Bank1( bad side ) goes from .7 to .9 volts when this is happening
Bank2 (good side ) is on .3 volts or there abouts expected

Interesting you should mention the heater. The monitor for oxygen sensor heater isn't coming up. I never thought about a lazy heater.

The sensors are original from 2001. I'll double check that I don't have any blown fuses for the heater and then check.

Originally Posted By: mk378
Exhaust leaks before the O2 sensor let air in, causing a false reading from the O2 sensor. The PCM will think it is running lean when it is not.


The confusing part is that it almost completly corrects itself after a minute or two. And I always thought at idle it would be a vaccum leak and at high throttle it would be the manifold. It definitely can throw it off.

I need to tow quite a distance with the truck in October, just a bit worried that the manifold won't solve it

 

[mk378]

Is it throwing any codes? At some point the PCM may give up on trying to fuel trim, ignore the O2 and revert to a safe mode.

You're seeing a major difference between the two sides, and the obvious thing is that one side has a leaky manifold.

 

[Trav]

A leaking exhaust manifold will give the O2 false lean readings.
In open loop this shouldn't matter so it may have an injector leaking down into the cylinder or other issue.
I would pull the plugs cold and see if any cylinder(s) is wet with fuel on the that bank.

 

[CT8]

The bad side has a high volts so wouldn't that be due to the great difference between the O2 inside the pipe and outside the pipe?

 

[CT8]

Originally Posted By: Trav
A leaking exhaust manifold will give the O2 false lean readings.
In open loop this shouldn't matter so it may have an injector leaking down into the cylinder or other issue.
I would pull the plugs cold and see if any cylinder(s) is wet with fuel on the that bank.
Excellent as usual Trav. The basics are always the starting point.

 

[SwampSurvivor]

In open loop it runs great. It will run great for as long as it takes to hit closed loop, then it will run poorly. If I shut it off and start it up again, it runs great until it hits closed loop again.

Sounds like it's not a massive vacuum leak. That's good. I'm going to see if I can temporarily patch the exhaust with furnace cement or something and check if that helps with this issue. I WILL be replacing the bad manifold, this is just a temporary fix.

Re: Plugs - it's a 2 valve modular. I'm not touching the spark plugs! haha. It has all 8 right now, I'm sure whatever I touch will spit out.

 

[SwampSurvivor]

Originally Posted By: mk378
Is it throwing any codes? At some point the PCM may give up on trying to fuel trim, ignore the O2 and revert to a safe mode.

You're seeing a major difference between the two sides, and the obvious thing is that one side has a leaky manifold.


No codes. I don't know how much before it will set a lean code.

 

[Trav]

Originally Posted By: Miller88
In open loop it runs great. It will run great for as long as it takes to hit closed loop, then it will run poorly. If I shut it off and start it up again, it runs great until it hits closed loop again.

Sounds like it's not a massive vacuum leak. That's good. I'm going to see if I can temporarily patch the exhaust with furnace cement or something and check if that helps with this issue. I WILL be replacing the bad manifold, this is just a temporary fix.

Re: Plugs - it's a 2 valve modular. I'm not touching the spark plugs! haha. It has all 8 right now, I'm sure whatever I touch will spit out.

Your call.

 

[SwampSurvivor]

The wires to the oxygen sensor on the side with the bad manifold (and the side that floods itself out) are very corroded. My eyes aren't good enough to see the color, but one of the wires has corroded away completely. I bet that's the one for the heater.

Now to get the sensor out of the 18 year old downpipe.

 

[mk378]

A broken wire at the sensor really should throw a code. Are you sure the CEL works?

 

[SwampSurvivor]

Originally Posted By: mk378
A broken wire at the sensor really should throw a code. Are you sure the CEL works?


This truck isn't big on setting codes. The CEL does come on for 3 secondss as part of the self check, but it doesn't set codes. Such as the time it blew a coil and we had to drive it 2 hours with a dead cylinder. Or the time the coolant temperature sensor came unplugged. Or the time there was a hole in a piston and it had a pretty severe miss.

It knows that the oxygen sensor heaters aren't working, but it won't set it. Also, the values it's getting for the B1 S1 oxygen sensor should set a lean code at this point.

At the time (2001 MY) there were much, much less stringent requirements for something with this high of a GVWR. It has an EVAP system but it's not monotired. One of the charcoal canister lines rusted through and I capped it on both sides of the hole. No codes or anything.

 

[SwampSurvivor]

I changed out the oxygen sensor. The wires were definitely bad and shorted together.

There is a fuse for the heaters (HOS / Exhaust Gas sensor) but it's not blown. The monitor still hasn't come up for the oxygen sensor heaters.

However, 2 days now it has started and run pretty well. This morning was 68 degrees, but yesterday was 75 degrees. Both should have been warm enough to cause it to run very poorly. There's still 6-10% positive fuel trim, which seems much more inline with the exhaust leak before the sensor. Hot idle is much more smooth now.

I'm planning on having the manifold changed next month. And if I didn't have to fight with this one to get it out, I would probably replace the other oxygen sensor. I imagine at its age, it's also getting tired even if the wires are still intact.

 

[SwampSurvivor]

I think the oxygen sensor confused the computer enough for a few days that it didn't do it. After a few cold start cycles, it started doing it again.

I had the exhaust manifold replaced but it was too cold for this to happen again.

Now that it's finally spring, I am no longer having the problem with the cold start issues. I'm fairly confident that it was 100% caused by the exhaust manifold. The ear was broke off for the bottom bolt of the #4 exhaust port. Along with it being almost completely rotted through between 1 and 2.

I wonder if all the other Ford trucks with blown out exhaust manifolds have the same problem? Probably 50% of these trucks sound just as bad as mine did.