1990 Ford F150 5.0. My rpm's were 1400 to 1500 at idle then would drop to 700 to 800 about 30 seconds later. I changed the plugs, wires, cap and rotor. I had a light show under the hood from the plug wires. I put a second set of plug wires on, now there is no light show. I followed the firing order of 1-5-4-2-6-3-7-8. Counter clockwise on the dist. cap. Now after I did changed the plugs, wires, cap and rotor my rpm's are 900 to 1000. When I put it in gear the rpm's drop to 400 to 500. Can someone help me out please, and thank you in advance, Brian

 

So it now holds a 900-1000 steady idle under no load and drops to a 4-500 steady idle when you load it down with the torque converter? If that is the case, I don't see what the problem is.

 

Spray carb cleaner just behind the distributor at the base of the upper intake plenum. If the engine revs up, you have a vacuum leak due to a faulty plenum gasket. If you go to the dealer and get one for a 92-96, it will be made of a plastic type material and doesn't blow.

 

I agree with SR - sounds close to normal. In this rare case, I'd kill the KAM by either disconnecting the battery, disconnecting the EEC, or pulling the KAPWR fuse to clear the adaptions. For detailed info, read the thread linked in this caption:

 

Hey guys, I cant thank you enough. I went down to the Mopar Nats this weekend. I just got the messages today Maonday the 16th. I took the truck in and the shop said I have a 5.8 block with a 5.0 plenum. I dont know if that is possible so maybe you guys could shed some light on this. Again guys your awesome. Have a great week, Brian

 

That's news to me, then.

 

The lower intake is specific to the engine because of the difference in deck height. A 5.0 lower will not fit on a 5.8 block. I was under the impression that the upper manifolds which contain the plenum were the same for both engines

 

AFAIK, the 5.0L & 5.8L (non-L) truck plenums are interchangeable. I think it's only the words on top that are different. The 3 possibilities are "EFI", "5.0L", & "5.8L".

. .

The L intake (upper & lower) is very different from those, but similar to the cars'.

 

Brian, your 1990 5.0 should cold start with a high idle at about 1400 rpms, hold for about 30 seconds and then drop back to 750 rpms no load. When you drop it in any gear, the IAC is suppose to compensate for the extra load and the idle should stay at 750 rpms. If you bought the truck used and it's had this problem, the previous owner may have turned up the throttle plates at the throttle body because the IAC isn't working. I'd suggest taking the throttle body off of the engine to clean it and make sure that the carbon is removed from the balance port just inside and behind the throttle plates. Make sure the port to the IAC is clean and clear. Remove the IAC from the TB and clean it. DO NOT use any throttle body cleaner on the TB while it's on the engine unless you want to destroy the plenum gaskets which will lead to a vacuum leak and a high idle. Reinstall it and then see what you have. You may have throttle plates that have carbon buildup on the back side giving the high idle and the IAC may be out of range due to the carbon. You won't know until you clean it but it must be removed from the engine to clean. If that doesn't work, look for a vacuum leak especially broken hoses. They're brittle and subject to break with their age. Look especially at the vacuum canister at the front left side of the truck. There is a plastic conversion piece that it subject to break in the hose. I'd also suspect the IAC is toast but clean it first as it may just be carboned up.

 

Ok guys thank you again. Im going to type down here what my bill sayd word for word. The truck runs great after they did the work. Here is what the bill says

ANALYZE RUNNING ROUGH
CHECK FOUND FIRING ORDER
CORRECT FOR 302/5.0L V8 ENGINE
AND TDC OFF. INSPECTION-FOUND
ENGINE BLOCK IS A 351/5.8L.
CORRECT FIRING ORDER FOR
CORRECT ENGINE, RECHECK AND
ROAD TEST- NO OTHER PROBLEM
FOUND AT THIS TIME.

That is what the bill invoice says. The motor was replaced before I bought the truck. It ran awesome for the first 3 months. I had the problem I mention, took it in and it gone. I didnt know what else to do so I took it in. I dont like dealin gwith auto shops being some try to take you for your money. Thank you again guys. Have a great day

 

If the plenums are interchangeable, then why do they have different port configurations? The 5.0 has a "square" hole plenum and the 5.8 has an oval hole configuration. Physically they will "bolt on", but will not work due to the different port configurations.

 

I've never compared the port shapes. If they're different, then they're not interchangeable.

 

thank you again guys. The truck has been running good since I picked it up from the shop. I cant afford to take it in again to get it checked out any time soon. But once I get the cash I will. I want to keep this truck on the road as long as possible and dont need the parts bill. Im doing my best. Thank you all again.

 

does anyone out there know if there is a front receiver sold by a company for these old fords? id love to get one but cant seem to find one. thanks for any help.

 

What does that have to do with a 5 year old thread discussing the 5.0 engine?

Please start your own thread. Thank you.