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idle miss/shake

 
  #1  
Old 07-09-2019, 06:27 PM
hawkco66
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Question 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.
 

Last edited by hawkco66; 07-09-2019 at 06:38 PM. Reason: more info to add
  #2  
Old 07-13-2019, 05:08 PM
baddad457
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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.
 
  #3  
Old 07-18-2019, 02:19 PM
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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?
 
  #4  
Old Yesterday, 12:34 AM
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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
 
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Old Yesterday, 09:06 AM
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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?
 
  #6  
Old Yesterday, 04:40 PM
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Originally Posted by EllieMae94 View Post
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.
 

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