idle miss/shake
 

I have a 1994 F150 with 5.0 and automatic trans. I've owned it since it was new. My daughter now drives the truck as her daily driver. I rarely drive it, but was riding with her a few weeks ago when I noticed a significant feeling similar to a cylinder misfire at idle while in gear at the stoplight. The engine never stalled, nor seemed affected in performance in any way.

Truck history:
The truck has 210K miles, but I replaced the motor with a Ford Reman long block about 35K miles ago. During the motor swap I replaced fuel injectors, plugs, wires, distributor, cap/rotor. I replaced every sensor during the swap except the knock sensor (I couldn't find one during my time window. Since that time I've replaced the factory A.I.R. pump, PS pump, alternator, and all vac hoses.

Yesterday I started some troubleshooting by beginning with a basic tune-up and checking a few vitals. I replaced the distributor cap/rotor, plugs, and air filter. With the plugs out I performed a dry (and cold) compression check (all plugs out). I started on the passenger side and got about 170psi plus/minus 5psi of each other. Plugs pulled looked totally normal. no carbon at all. Plugs coming out and going back in were the same; motorcraft platinum factory plugs. Some wear was on the electrodes--not excessive wear. The cap and rotor were the same. In the old days I would have used some sandpaper to clean the contacts and re-used them, but now they are new. I worked the driver side next and got 180psi plus/minus 5psi on all four cylinders. All the plugs coming out looked as good as the other side. No one single plug looked any different than the rest.

I performed an idle fuel pressure check and the needle held steady 30psi.

With engine off I used handheld vac check and the fuel press regulator held vac with no leaks.

I connected a vac gauge to the intake and while at idle and warm i was pulling only 17psi, but the needle was rock steady, even while the "miss" was occurring. I removed and plugged the vac lines feeding the AC and emissions to see if the idle smoothed out--no change.

With everything reconnected and engine running at idle with the "miss" occurring I disconnected the IAC. The engine RPM decreased significantly, but the miss remained.

With engine running at idle I sprayed carb cleaner all over the intake and other likely areas a vac leak could occur--no change in rpm.

I removed the breather to inspect the throttle body plates. They were very clean and with the throttle open there was only normal carbon beyond the throttle plates.

The truck has OBD1 ( EEC-IV) so not the best computer to help find the problem. There are no codes for engine off or engine running. All systems pass.

The large disparity in compression from the left bank to the right bank bothers me so I will recheck at least two cylinders on the right side tomorrow to see if somehow I goofed that up.

So, do I have:
1. bad plug wires (only 35k miles/3years on them), but I will do the night-time water vapor spray to look for arc later tonight.
2. will the coolant sensor cause this if faulty (and not tripping a code), (Plus the gauge seems to read fine).
3. Poor fuel quality---fuel has been replaced at least once since I first noticed this (but this problem has likely been there for a while)
4. burnt exhaust valve even though one cylinder compression check doesn't reflect major difference from others around it and the (although low) vacuum doesn't show shaking needle---rock steady.
NOTE: there is zero pressure coming into or out of the oil fill cap on the valve cover with engine at idle and warm--so I assume no burnt intake valve.
NOTE: hard to really tell if the miss is causing a flap of paper to suck at the tailpipe or if the miss causing a drop in RPM stops pushing the paper--not discernable.
5. other ideas?

thanks for any help.

There's no knock sensor. So you can eliminate that. You sure you go the correct short block ? The camshaft changed in 94, the earlier version used a different firing order.

A '94 with a 5.0 will have the knock sensor, at least mine has it. Did they not come on remanufactured blocks?

17" at idle isn't too bad, are you at a higher elevation?

Did you check the timing with the SPOUT pulled out?

Seems to be hit or miss with the knock sensor. Had a friend with a 92 E150 had one. Neither of my 95 and 96 E150's with 351 had them. Nor did a 92 F150 or a 95 F150 have them. The 92 was a 302, the 95 a 351

Yeah, another "Fordism" thing I suppose. From my understanding no 351's had them, only the later 302's with a roller cam did?

OP, did you get this figured out?

I've only seen that one 302 that had one. It was either a 92 or 93 van. The 92 pickup I had was also a 302 but no knock sensor. Some roller 302 blocks are drilled and tapped for it, some aren't.

Thank you for your ideas.

There was definitely a knock sensor. I'm in Texas so I'm pretty close to sea level around 200 to 300 feet.

I've got some more time to look at it this week, so I'm going to re-verify some of the vitals. I will recheck timing with Spout removed as well as EGR. I plan to remove the throttle body for inspection. When and if these check out I will remove the upper intake plenum for access to the injectors. Next will be injector removal/inspection.

I've replaced the PCV.

I removed the EGR valve and bench tested it. It looked to be in good shape. Inspection revealed the same. I checked the ignition timing with spout removed and again with it installed. With the spout removed I'm reading dead on 10 deg BTDC. With spout plugged in it advances further to about 14 deg BTC. I'm thinking that passes ok.

The shake/miss is becoming worse. We've stopped driving it until I can pull the fuel injectors for testing and also perform a leak-down test to find a cylinder with a problem...

Update:
Truck rolled in and out of the carport as other projects took over.
My truck's very rough idle remains unsolved.

I removed the upper plenum and valve covers for inspection. No obvious problems. All looked good. I installed new stock injectors and new valve cover gaskets. With the valve covers off I performed a leak down test. This was the first time I'd done this test. I expected my leakage to be close to zero and was hunting for a large leak outlier to point too for my trouble. However, I found leakage on all cylinders. None exceeded 20%, but I ranged from 10% to 20% across the board. Felpro gaskets were installed on the upper plenum. Throttle body was already clean so I put in a new gasket and reinstalled it. The engine ran as it did before. Rough idle. In fact, when in gear it lopes like a car with a very hot camshaft---"lug lug lug."

So, I decided to replaced the EGR valve in case it was not springing completely closed. I installed a motorcraft EGR valve. Problem remained, but I now received a code for the EGR sensor on top. I cleared it several times and it came back. I installed a new motorcraft EGR sensor and the code went away. Idle problem remained. I looked hard at the idle motor on the throttle body. In for a penny, in for a pound. New motorcraft idle motor installed. Problem remained. I pulled off the purge valve. It tested bad. My spirits were up. New Standard purge valve installed with new hoses. Problem remained. I looked at the TPS. It passed all ohm checks, but was not calibrated to 1.0v at idle. It was at .90. So I adjusted it to 1.0 volt. Ugh, problem remained.

I unplugged the MAF connector and started the engine. Oddly, it ran smooth, real smooth for 1 minute, then started its rough, surging, lopey idle again. With MAF plugged in, I unplugged the Air charge sensor that is plugged into the air cleaner box. The engine rain smooth for 1 minute then started running rough again. I performed some multimeter ohm/volt tests via backprobing on the MAF. It passed.

I have not looked at the coolant sensor yet. But my scanner shows a temp value of 190deg with engine hot on a hot day at idle. That sounds right to me.

So, is my PCM bad? or Ignition module? other?

I removed both the PCM and ignition module for a visual test. I opened the PCM (which is a remain). I replaced it around 10 years ago along with the ignition module. There was no sign of capacitor failure or resister burn-throughs. I cleaned it up and re-installed both. Idle problem remains.

On a whim, I replaced the O2 sensor. Idle problem remained.

Any other ideas? what am I missing?

Thanks for any thoughts,
Greg

In the past I've seen dirty injectors give an EGR code. The problem wasn't the EGR at all, but when it would cycle on, it would enhance the problem with the dirty injectors. Hooked it up to a Motorvac machine and it fixed the problem. You could also have a bad PIP sensor in distributor. And parts store distributors usually don't replace it when they reman them

thanks for the idea. I will look into that.