1990 F-150, 302, automatic, FI, 98K miles.
Problem:
Truck shakes/shudders @ 1000-1200 rpm, 25-35 mph, light throttle, worse under load. Vacuum drops to 5". Giving it either more or less throttle eliminates problem (mainly going into 2nd, and again going into D but much less severe). Will also do it @ 1200 rpm with truck not moving but much less severe. Slight ticking at idle though I'm not sure if it is a lifter or exhaust leak. Engine hot or cold, doesn't matter.
New:
plugs, wires, cap, rotor, air and fuel filter, PCV, oxygen sensor, drive shaft balanced, u-joints.
Checked:
fuel pumps - 21psi engine off, 34 running, 40 with fuel regulator vacuum disconnected. (which is what it is supposed to be)
EGR - when disconnected no effect on problem, when vacuum applied engine stumbles. Did get a code 33 ("not reading position of valve") but that could of been left over from before previous owner replaced EGR. Besides, disconnecting the vacuum to EGR should have solved problem if it was opening too soon.
Vacuum 'T' on manifold - plugging ports no effect except runs worse.
Idle air control valve - disconnected makes it worse. 0 ohms at solenoid, 7 ohms across wires. Chilton says 7-13 ohms but do they mean valve or wires?
Also a code 21, temperature out of range, but that could be an old code. Besides, the problem is identical hot or cold engine.
Should also mention it wants to die when you first start it. Highway is great though.
Read somewhere about pushrod/rocker problem could cause this.
Anyone got any thoughts?
Thanks.

 

Have another look at the IAC.
Remove and clean it.
It doesn't take much crud to throw things out of whack.

 

Try replacing the EGR position sensor. $18 or so at Autozone.

 

Those both sound like good ideas. Will do that tomorrow and post results!
Thanks.

 

Sounds to me like you've got a couple of plug wires swapped.

 

Checked once but I'll check again.

 

Test the throttle position sensor for a smooth output. The reading should be less than 1V (.9V is good) at closed throttle, and the voltage should smoothly increase as you open the throttle, to around 4.8V at wide open throttle. The voltages are measured with the key on, engine off, between the green wire and the black wire on the tps while still plugged into the main engine harness. You'll have to back probe the connector to get a contact.

 

have you checked for codes. whats your timing set at? check for vacum leaks?

 

After all the tune ups I've done over the last thirty years, the plug wires still get me. Last time I tuned up the 351 in my 95 E150 van, I had a couple plugged in wrong.

 

Thanks for everyone's help, this is a great forum.

baddad457: plug wires correct

kemical burns: Looks like code 11, system pass. *=flash
*2sec*3sec*2sec*7 sec *6sec*2sec*3sec*2sec*
checked all hoses for intake leaks, but I think an intake leak would be worst at idle.
Timing is only 5 degrees, should be 10! (with that little jumper at distributor wires disconnected per decal info). Big problem? I do get advance with jumper installed running up RPM's.

EPNCSU2006: Throttle position sensor checked out perfect, just what you said.

ratstang: EGR position sensor is up to $26 at my Autozone! I know the EGR was replaced and the sensor looked pretty new, opened it up looked clean. I will hook up a vacuum gauge to it though and see if my problem occurs when EGR kicks in. Although I didn't notice any difference in performance with the EGR disconnected.

Stuart1:
Took apart and cleaned IAC. It was a little dirty and I think it moves a little freer now, but didn't produce a change.

A guy in Autozone (customer) suggested ignition module, but problem exists hot or cold.

More ideas?

 

It doesn't sound like the ignition module. If the module was bad, it is a rare occurance for the truck to run at all.

 

true.
I'm leaning toward air/fuel mixture (hey that's funny).
The problem is just way too influenced by throttle position and load to be electrical. At least that's my thinking. Part and WOT at highway speed is fine.

 

I'd be looking at the timing. With 5* less advance that could eat lots of ponies.

 

the timing is a big factor. set that at 10* btdc and you will see a big difference

 

Well you were both right. Put the timing at 10 degrees and what a huge difference. Feels like the truck is twice as powerful. A manual I have said you couldn't adjust the timing on a '90!

Still have original problem but it is confined to a much smaller band of operation. It's still pretty severe shaking just after gear change at around 25 or 35 mph.

At idle I can feel roughness @ 1100-1200 rpm (with 17.5" vacuum, vacuum at curb idle is 16" @650rpm). Idle @ 1000 and 1300 are fine though.
Idle air control tests okay, 7V or 13V, (depending on which terminal you put the red and black on), valve moves freely, but I can't help but think it has to do with air delivery/mixture. What's a good way to test for intake manifold leak? I never had any luck with the propane method and the way the manifold is stacked up like that I don't know how I could squirt oil in there to seal it.
Nobody thinks it could be valve/pushrod/rocker problem? I'm saving the fun stuff for last. I notice my vacuum gauge seems to flicker in time with the slight ticking I hear. I guess a valve problem would show symptoms over a much wider band of operation though.