i have a 90 150 4x4 w/ a 302 efi. in 4th gear going uphill when the motor gets under load it starts to stumble like it is not getting enough fuel or the timming is not advancing or something. i changed the timming chain and gear out 3 months or so ago. i am starting to wonder if i am not a tooth off. been having spark nock and problems since. i have moved the distrubitor around form one end to the other w/ not much help, and also took it out and moved it one tooth over. i have checked tdc and that is perfect. i have got good power 1st 2nd and 3rd at low rpm's there is a rpm range in 3rd around 3000 or so it does'nt like to. i am at my wits end any suggestions of things to try or do i would apperciate it thangs

RobertS

 

Check your fuel pressure and check for codes.

 

fuel pressure is ok according to local m shop. could bad or worn out injectors have this effect? the codes point to map sensor, but when i take a volt meter reading it says it is ok. i also unpluged it to see if would make a difference i could not drive it would not run. thanks for the help

 

Have you done a compression check? Having this problem under load sounds like you may have one or more burned exhaust valves. Good luck.

 

i done a compresson check write after i started having the problem. i changed the plugs and checked it then. some body mentioned to me that i might could but cooler plugs in and that might help. i change the 02 sensor last night and reset the computer and the end of the sensor was real white if you can read them like a plug i might be running real lean. i don't know if this could be part of the problem or not.

 

It sounds like a timing issue to me. Like you first said, " could be off a tooth". Pain in the ***, but that's where I'd look. Like some one said about the exhaust valve, the ill placed cam position will definately cause this type of problem, especially under load.

 

thanks everbody for the help will check the timming chain and let you know what i find

 

To answer your question about whether you can "read an O2 sensor like a spark plug" the answer is not really, no more than a tailpipe. Having said that white/light grey is normal where as black/dark would indicate the O2 is dead and has been sending a lean signal to the computer causing it to run rich. Obviously if it is oily then your engine is burning oil. So far as your original complaint, I agree with the others I think your problem is base timing.

 

I think you need a scope to read the Map correctly, even if the voltage is ok. I think on the 87 to 89 just a voltage check is ok. I heard if you unplug the sensors and it acts different its maybe that sensor. I'm no expert. I have an email about how to check your map with a scope, i try to find it since the search function is out.
87 f150 4.9 efi
mike
from an old post
"I found out yesterday that my multimeter has a frequency tester on it, which can be used to test the MAP sensor (I did it yesterday myself just to see if the multimeter worked for that). You don't need a scope."

 

I would add a bottle of fuel injector cleaner to the gas tank if you haven't already tried this. I would also consider changing the spark plug wires, distributor cap and rotor.

 

If the cam timing is off, the engine will run smooth enough but be down in power. It won't feel like a "stumble".

A failure to advance timing (as by leaving out the SPOUT jumper) will cause the engine to be down in power, but not to miss or stumble. Excessively advanced timing will cause pinging and detonation under high-load conditions, but it doesn't sound like you have that.

Check the indexing of the rotor to the spark plug wire tower with the engine at 10 BTDC on #1 compression stroke. It should point towards #1 plug wire tower, not halfway in between two towers. An "off by one tooth" distributor could possibly cause a stumble under load, if the path for high voltage through the dizzy is too circuitous.

Your fuel system may deliver adequate pressure statically, but fall apart when the engine is placed under load and fuel demand is high (as in "4th gear going uphill").
This would cause a miss or stumble when the engine is asked to produce the most power. Kinked fuel lines or a blocked filter sock in the tank will do this. So will a fuel pressure regulator that fails to react to lower vaccuum/higher MAP. (You did check that the vaccuum line is connected to the regulator, and that fuel pressure drops when you start the engine, right?)

The easiest way to diagnose fuel volume problems is to rig up some way to measure fuel pressure while going down the road.