79 400 knocking question
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Joined: Dec 2004
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From: South Carolina
79 400 knocking question
I have a 79 F250 4x4 with a 400 & granny 4sp. Motor runs pretty good power is not great but pulls my 10,000lb trailer quite well. The knock I have is weird. It only knocks when I am at a crusing speed above 1500rpms. It sounds like a diesel. If I give it throttle it goes away and drives normal. If I let off, the rattleing quits. It has done this since I have owned this truck. I have an aftermarket oil pressure gauge on it from the previous owner, so I don't know how accurate it is. Pressure seems fine at start up, goes down as it warms up. I have not been easy on this motor, towing everything under the sun & four wheeling in the mud and this motor has not come apart yet. But I don't know how long it can go on like this. Does anyone have any ideas what this could be, any ideas would be appreciated, Thx in advance.
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From: South Carolina
I know my carb is good, haven't had any trouble out of it except having to use a little shot of gas to get it going after letting it sit for weeks. Don't think I've messed with the timing too much. It starts right up almost everytime. once it has been started for a minute, it fires off with just a quick turn of the key. The rest of the drivetrain is solid, body not so great but it has been a great truck save for this one problem. It has me thinking of selling it and getting another truck. Hate to do it but I don't want this to cause a problem when I'm hauling 10,000lbs behind it. I'm to the point I won't drive it any farther than around town.
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Joined: Aug 2001
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From: Delaware
Is your EGR valve not working or disconnected?
That is a typical symptom of EGR disconnected. The carb is set up for a certain amount of exhaust gas to be mixed with the air/fuel mixture from the carb. If the EGR does not open, then there is no exhaust gas in the mixture and it is too lean. This lean mixture detonates and causes the audible ping. When the carb is fully opened, the mixture is OK and the pinging goes away.
That is a typical symptom of EGR disconnected. The carb is set up for a certain amount of exhaust gas to be mixed with the air/fuel mixture from the carb. If the EGR does not open, then there is no exhaust gas in the mixture and it is too lean. This lean mixture detonates and causes the audible ping. When the carb is fully opened, the mixture is OK and the pinging goes away.
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Tuned
Joined: Jul 2003
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From: Berkeley
you may have too much vacuum advance. my 400 was pinging under load at partial throttle. backed off the vacuum advance a bit and it went away.
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From: Hartland, MI
You didn't mention the PSI reading on the gauge? I think you have a main bearing issue, they will knock when unloaded, such as at a steady cruise. If you hold RPM's at about 2500 in nuetral what does it sound like? If it was pinging you would hear it under load.
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From: South Carolina
Worked on it a little today. Vaccuum advance is hooked to a timed port. 25lbs at throttle.
EGR is present and looks hooked up, but there is no vaccuum line running to it.
The oil pressure reading is 60 at start up, but goes to below 20 when warm. Anything over 2000 or so and the motor rattles. Hope it is not a main bearing problem, that is my worst fear. If it is I will be forced to sell it and buy another truck as I cannot afford the cost or the time of a rebuild.
What about "rolling" a set of bearings in it? Anyone ever done this?
EGR is present and looks hooked up, but there is no vaccuum line running to it.
The oil pressure reading is 60 at start up, but goes to below 20 when warm. Anything over 2000 or so and the motor rattles. Hope it is not a main bearing problem, that is my worst fear. If it is I will be forced to sell it and buy another truck as I cannot afford the cost or the time of a rebuild.
What about "rolling" a set of bearings in it? Anyone ever done this?
Tuned
Joined: Jul 2003
Posts: 455
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From: Berkeley
sound's like you need to either hook your EGR up to vacuum (ie: fix it) or re-curve your dizzy to compensate for no EGR...
btw: you can tell if your EGR valve works by applying vacuum to it (ie: suck on it) at idle. if it bogs down, it's working. if sucking on it does nothing, the valve isn't working or clogged up somehow...
hope that helps!
btw: you can tell if your EGR valve works by applying vacuum to it (ie: suck on it) at idle. if it bogs down, it's working. if sucking on it does nothing, the valve isn't working or clogged up somehow...
hope that helps!
Last edited by packlet; Dec 19, 2004 at 10:40 PM. Reason: typo
Senior User
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From: Hartland, MI
You've got bigger problems than EGR or timing. If it rattles at anything over 2000, you've got excessive clearance somewhere. Sounds like its the mains, but without hearing it I can't be sure. I have rolled bearings in on a 2wd truck with a 351M and it wasnt too bad. I think a 4x4 would be even easier. The hardest part is replacing the rear main seal.
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From: Hartland, MI
I just used standard bearings since that was what I had. It is very hard if not impossible to accuratly measure the journals when the crank is still in. I put the engine back together with new rod and main bearings and a high volume/high psi oil pump. The engine had MUCH better oil psi after the repairs and no noises at all.
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