Losing throttle response intermittently

Put a new battery in the truck not because it was bad, but it was 5 years old and from the previous owner. Wanted to get a fresh one in. Battery holds charge, gives ~12.5v on the meter when off and ~14.5 when on. This issue started immediately after changing the battery but I don't think it's related but wanted to mention anyway.

The issue:

When driving at low speed, usually in 1st or 2nd (manual), the truck will surge or hesitate (feels like driving with someone who is learning how to drive a manual) and the gas pedal will do absolutely nothing for a short period, it's very apparent at low speed. When I am in 4-5 gear it does not surge or hesitate in the same way but again the gas pedal does nothing.

This can happen all the time (its nearly undrivable) , or infrequently (once in a 10 minute drive), can happen immediately at the start of driving or start happening a few minutes later.

I thought I'd get lucky when I changed the fuel filter since I hadn't since I bought the truck in May. No change. Gave it sea foam for the first time through the fuel and intake and again no change.

I opened the ECU and did a visual check for leaky caps. They *look* 100% fine to me but I am going to change them ASAP anyhow since cap failure is not always visual.

Whaddya think?

Here are two videos to illustrate. Only the first 30-60 seconds are relevant.

Trouble starting the vehicle (may or not be related): http://1drv.ms/1HgHV9O
Example of the behavior described above: http://1drv.ms/1HgI0ue

Hope that helps!

Sounds like a stuck valve causing it to missfire at startup ? That would be reason for loss of throttle response when driving. Just my guess with out driving the Truck to see.

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Originally Posted by maxdsm

Sounds like a stuck valve causing it to missfire at startup ? That would be reason for loss of throttle response when driving. Just my guess with out driving the Truck to see.

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Interesting. All I can say is that the truck has had trouble starting up since I bought it in May and just recently did the acceleration issues arise. Do you still think they would be related based on that?

Check all of your points on the distributor,rotor button plugs and make sure you are getting proper fire at the plugs first to rule that out.

Repost.

Cap, rotor, wires, and plugs were newly installed by me in May but I will certainly check. Thanks for your advice!

Confirmed that spark is good at the plugs just now and made sure everything is connected well enough. The sparks are looking blueish-purple and I didn't see any misses - then again, I really only experience this problem while driving.

Also did a quick diagnostic test for a stuck valve I saw on Youtube which prescribed holding a piece of paper in front of the exhaust and looking for it to be sucked toward the pipe while running. The paper wasn't being sucked toward the tailpipe at all. Not sure what else I could look for with respect to a stuck valve without taking things apart.

Take off the rocker cover, pull the plugs and turn the crank by hand with a socket and extension bar. If you see any slack on the rockers as you turn the engine you have found your dud valve. You might have to grab the rockers and jiggle them by hand to feel for play if you can't see it. It is easy enough to do and is quick. Other options are borrow a compression tester from a friend and see what prevails and/or just look at each individual spark plug and see which one stands out from the rest.

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Originally Posted by geezer101

Take off the rocker cover, pull the plugs and turn the crank by hand with a socket and extension bar. If you see any slack on the rockers as you turn the engine you have found your dud valve. You might have to grab the rockers and jiggle them by hand to feel for play if you can't see it. It is easy enough to do and is quick. Other options are borrow a compression tester from a friend and see what prevails and/or just look at each individual spark plug and see which one stands out from the rest.

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Thank you for the clear instruction. I will do this but it might have to wait a few days as I'm going to be visiting family. Stay tuned!

Another suggestion would be to use some Carb cleaner to clean your carburetor, then raise your idle screw by just a tiny bit. Let it dry, then see what happens

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Originally Posted by MightyMax1991

Another suggestion would be to use some Carb cleaner to clean your carburetor, then raise your idle screw by just a tiny bit. Let it dry, then see what happens

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I'm fuel injected :)

Waiting for some valve stem seals to arrive since I've got smoke at startup. I'll be looking for stuck valves while I install those.

A couple weeks ago, since the weather is cooler, I was able to determine that the issue doesn't happen when the truck is at op temp. Leading me to believe a stuck valve is the case here.

It doesn't happen when it's up to temp? then you may have a problem with the engine temp sender. The ECU adjusts fuel and and timing based on air temp and engine temp (and a lot more, but I digress). This is a different sender than the one that drives the gauge on the dash, it's the little one on top of the water pipe where the radiator hose attaches.

I see the sensor you're referring to. I'll see if I can find any info on testing its functionality before I pull it off. Thanks for an additional theory.

my 1990 used to do this, ended up being the ecm. go ahead and check it to make sure their are no leaking capacitors or fried components. the throttle would cut out randomly and when it got humid out it would not start.

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Originally Posted by finalfighter

my 1990 used to do this, ended up being the ecm. go ahead and check it to make sure their are no leaking capacitors or fried components. the throttle would cut out randomly and when it got humid out it would not start.

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This is on my to-do list, and it's been in the back of my mind too. However, visually the caps look OK. The ones that tend to leak are the original Rubicon (iirc) and I'll be putting some new ones in after I get my work under the valve cover finished.

When your '90 did this, did it do it all the time or before it reached running temp?

cold start it would have a hard time trying to stay running. it would mostly idle really low, shake, and miss a bit. once it warmed up it would fire right up or not at all. when driving down the road the throttle would stop working at random hot or cold-(if i could get it down the road when could).

While we are talking about coolant sensors, do I have these right?

Attachment 15551

1. ECU
2. Cluster gauge
3. A/C