Slow Bronco
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Fleet Mechanic
Joined: Aug 2001
Posts: 1,526
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From: Round Rock, TX
Slow Bronco
I've got a problem with the '92 351W. I've got a slight lope at idle, and I'm getting less kick out of this engine than I remember getting from it in the past. It's got fresh filters, plugs, cap, rotor, wires. I've hooked up a vacuum guage and run the engine at about 3000 RPM with no drop-off in vacuum, so I'm kind of thinking that it's not an exhaust restriction.
I do have a problem with #5-the plug gets oil fouled every 3-4k miles and I have to replace it. I'm thinking that there's a worn valve guide on that cylinder, which would possibly explain the loping idle and a regular but slight vacuum drop during idle. However, I'm thinking that this is a separate issue than the lack of power.
Greystreak mentioned replacing the knock sensor, but is this necessary if I'm not getting any codes? If it could go bad w/o throwing codes, then I may consider replacing it. I've replaced the O2 sensor w/in the last 20k miles, but should I do it again?
I do have a problem with #5-the plug gets oil fouled every 3-4k miles and I have to replace it. I'm thinking that there's a worn valve guide on that cylinder, which would possibly explain the loping idle and a regular but slight vacuum drop during idle. However, I'm thinking that this is a separate issue than the lack of power.
Greystreak mentioned replacing the knock sensor, but is this necessary if I'm not getting any codes? If it could go bad w/o throwing codes, then I may consider replacing it. I've replaced the O2 sensor w/in the last 20k miles, but should I do it again?
Senior User
Joined: Mar 2003
Posts: 142
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From: Mechanicsville
Slow Bronco
One weak cylinder will kill your torque. Instead of the culprit cylinder making torque it has to be carried by the other seven cylinders. Could be a valve seal, valve guide , broken ring , or cracked piston causing the plug to foul. A compression test of all eight cylinders is your best bet at determining engine condition. When performing the compression test remember to disconnect the coil wire and wire your throttle wide open. Have a great day. Broncomule
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
I haven't done a compression test yet; I'll probably get to it this weekend. The engine was a rebuilt engine from ATK that was put in about 70k miles ago. It was about 30k-40k miles into its life when this problem developed.
I'll do a compression test and post any interesting results back to the forum. I'm sure that #5 will yield some good data. I'm still seeing references to a clogged cat--is there another test to perform besides the vacuum test and besides cutting the exhaust up? If I'm going to cut the exhaust I may as well replace the cat.
I'll do a compression test and post any interesting results back to the forum. I'm sure that #5 will yield some good data. I'm still seeing references to a clogged cat--is there another test to perform besides the vacuum test and besides cutting the exhaust up? If I'm going to cut the exhaust I may as well replace the cat.
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
OK, I did the compression test last night, and the good news is that every cylinder is between 120 and 130. I guess that means that the oil fouling is probably just a stem seal. On the other hand, it doesn't explain my loping idle. I did replace the cap and rotor while I was in there last night, and it actually made a huge difference in performance.
Which brings me to another issue--the tip of the rotor gets very burned up and the towers in the coil get cooked and the entire thing gets filled with dust after about 10k miles, forcing me to replace the cap and rotor quite frequently. Any ideas?
Where should I look next for the lope in the idle? I'm not getting a "Check Engine" light, so I don't have any codes to pull.
Which brings me to another issue--the tip of the rotor gets very burned up and the towers in the coil get cooked and the entire thing gets filled with dust after about 10k miles, forcing me to replace the cap and rotor quite frequently. Any ideas?
Where should I look next for the lope in the idle? I'm not getting a "Check Engine" light, so I don't have any codes to pull.
Elder User
Joined: Mar 2003
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From: Medford, OR
Slow Bronco
You may still have codes to pull. There are "silent" codes that the computer stores, but that it doesn't think are important enough to set off your check engine light. Might be a good idea to pull the codes just to be sure.
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
OK, here are the codes I've pulled:
173 - O2 lean
452 - VSS problem
173 - O2 rich
452 - VSS problem
About the O2 problem-is this indicative of a bad O2 sensor, or is this just saying that the O2 sensor picked up a bad condition that could have caused by something else? Should I go ahead and replace the O2 sensor while I'm looking at all this?
I replaced the VSS sensor about a year ago because I was trying to resolve a dead speedo problem, which turned out to be a bad PSOM. The bad idle has existed for a lot longer than that. Should I go ahead and replace the VSS again?
In another interesting twist, when I was doing the thermostat last night, I looked into the intake through the throttle body and it looked quite oily in there. It looks perfectly clean before the throttle plates, but behind that and all the way to the back of the plenum it looked black and oily. Should I shoot some EFI cleaner in there to clean all of that out?
173 - O2 lean
452 - VSS problem
173 - O2 rich
452 - VSS problem
About the O2 problem-is this indicative of a bad O2 sensor, or is this just saying that the O2 sensor picked up a bad condition that could have caused by something else? Should I go ahead and replace the O2 sensor while I'm looking at all this?
I replaced the VSS sensor about a year ago because I was trying to resolve a dead speedo problem, which turned out to be a bad PSOM. The bad idle has existed for a lot longer than that. Should I go ahead and replace the VSS again?
In another interesting twist, when I was doing the thermostat last night, I looked into the intake through the throttle body and it looked quite oily in there. It looks perfectly clean before the throttle plates, but behind that and all the way to the back of the plenum it looked black and oily. Should I shoot some EFI cleaner in there to clean all of that out?
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
Replaced the O2 sensor, TPS, and fuel filter. Improvements in overall power, but I still have the poor idle.
Could oil seeping in through the #5 valve seals make the idle poor and kill power? As I mentioned in another thread, I've got a dying harmonic balancer, but that should just cause vibration, not cause the idle to fluctuate, right? I've used up all of my spare time for the next few weeks, so doing the stem seals will have to wait a bit, but I wonder if that would possibly fix the idle.
Could oil seeping in through the #5 valve seals make the idle poor and kill power? As I mentioned in another thread, I've got a dying harmonic balancer, but that should just cause vibration, not cause the idle to fluctuate, right? I've used up all of my spare time for the next few weeks, so doing the stem seals will have to wait a bit, but I wonder if that would possibly fix the idle.
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Thread Starter
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Fleet Mechanic
Joined: Aug 2001
Posts: 1,526
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From: Round Rock, TX
Slow Bronco
The plug wires are a set of Autolite Professionals that I installed a couple of weeks ago. It had Motorcraft wires before that, and they all showed normal wear when I took them off. When I pull the plug after a few thousand miles, it's got lots of white oil ash built up on it. If I wait a few thousand more, when it's really running rough, the plug is oily and wet. I definitely need to change the stem seal to correct the oil fouling problem.
I did shoot some EFI cleaner into the plenum when I had the throttle body off yesterday, but it didn't really cut down on the oil buildup. The PCV valve is replaced regularly, and is relatively fresh right now. Interesting...
I did shoot some EFI cleaner into the plenum when I had the throttle body off yesterday, but it didn't really cut down on the oil buildup. The PCV valve is replaced regularly, and is relatively fresh right now. Interesting...
Lead Driver
Joined: Sep 2002
Posts: 9,182
Likes: 12
From: Gateway to the West
Slow Bronco
How many miles on that engine? The 302 and 351 engine suffer from exsessive blow by as they age. To the point that the breather filter in the bottom of the airbox has to be replaced almost monthly. I only mention this because when I replaced the 302 in mine, the intake was so jammed with oil and crud from the crankcase that I had to have it boiled...like a block...to get it clean. The upper plenum was ok with a bath in the parts wash sink but the lower was positively crusty!
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
The motor has about 80k miles on it. It was an ATK rebuilt motor, and I'm not sure if the upper intake was moved over from the old engine or not. The old engine had about 75k miles on it, but it died from neglect (not changing oil) before I took over maintaining the truck. The breather filter actually looks pretty decent; it has about as much dust as the air filter, and isn't really oily. I had the L valve cover off a few days ago to replace the gasket, and it was actually very clean in there, with a little crust over the #5 cylinder, the one with suspect seals. But I have seen some 302s with severely oil-clogged breathers.
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
Slow Bronco
Cleaned the IAC motor REALLY well yesterday when I replaced the TPS. I've replaced the fuel pressure regulator before to try to correct this, with no change. Hmm...
I once had another make of vehicle which contained a 350, and the EGR valve went out. What it did was it had an odd idle, and when you put it under light throttle and let off again, it would stall. Not exactly what is going on here, but I do have a mild stumble under light throttle.
One thing that I noticed and mentioned in another thread a while back is that the condensor next to the coil looks corroded around the ends. Could this be causing this problem?
I really appreciate everybody's help here. This problem has been bugging me for some time, but I'm getting quite serious about fixing it now.
I once had another make of vehicle which contained a 350, and the EGR valve went out. What it did was it had an odd idle, and when you put it under light throttle and let off again, it would stall. Not exactly what is going on here, but I do have a mild stumble under light throttle.
One thing that I noticed and mentioned in another thread a while back is that the condensor next to the coil looks corroded around the ends. Could this be causing this problem?
I really appreciate everybody's help here. This problem has been bugging me for some time, but I'm getting quite serious about fixing it now.