intermittent spark?

[sbc1116]

I have an 84 4x4 wagon with the 3ac motor. While driving on the highway it will stumble like it loses spark to one cylinder.
Once I accelerate a little it will pick it back up until I try and maintain a constant speed than we start the same thing all over.

It doesn't do this all the time and I don't notice it in town.
This car has been in the family since new and has 287000 miles on it.
I am also getting poor fuel mileage on a bad day it will be 17 on it's best day 22. This is a manual trans rig.

I have replaced the fuel filter-plugs-cap-rotor and the wires where replaced by previous family member.

Suggestions?

[keith]

You still have to replace those plugs about every 30k miles. Try Bosch Platinum +4 plugs, this engine seems to respond well to them for some reason. Also check your valve lash, very important.

[sbc1116]

Plugs have less than 5000 on them. I am not sure when the last time the valves where adjusted though.
They are a tappet type valve system right not a shimmed system?

[Mark]

I had that problem after I put new plugs in. It ran fine at idle, but seemed to be missing in a cylinder when I accelerated. I replaced the plugs again and it was fine. I'm guessing the store gave me the wrong plugs the first time or one of them had a ground in it.

[dlb]

welcome, sbc1116. based on the symptoms you described, i doubt it's intermittent spark. however, you can check for intermittent spark by starting the car, pulling each spark plug wire one at a time, and then holding the end close to a clean portion of the valve cover. keep your fingers away so you don't get shocked. you should see a consistent bright blue spark. do that with one hand and then use the other to increase the throttle since your problem seems to occur at part throttle. also, do this test while cold and then while hot. as electrical components break down, they can work fine while cold but act up once warm.

i think your problem is likely a vacuum issue. use the vacuum diagram on the underside of your car's hood and make sure all the vac lines are routed correctly. also make sure none are disconnected, cracked, or broken. then put a clean hose on each of the diaphragms (throttle positioner, choke opener, choke breaker, vac ignition advance, etc) and suck. make sure you cannot suck air through. if you can, the diaphragm is broken and causing a vacuum leak. disconnect the hose to that device and plug the ports until you can replace the broken stuff. it's possible that both of your vac ignition advance diaphragms (found on the bottom of the distributor) are broken and causing a significant vac leak at highway cruising since vac is applied to both of those under hwy cruising conditions. i've had tercs drive fine even with those diaphragms broken and leaking but since that's when you're having problems, it's something i would investigate.

also check your ignition timing. disconnect and plug the manifold vac advance hose when you do this. stock timing is 5* BTDC, most peeps here run at 18* BTDC because it gives slightly better performance and economy.

your fuel economy does suck. i just went over this on another thread so check it out here. i have since added incorrect choke setting to the list because if the choke is not wide open when warm, you will always run rich.

https://tercel4wd.com/forums/viewtopic.php? ... 044#p65499

[xirdneh]

also check the two spring loaded contact pins (i think they are solid graphite?) in the distributor cap
if one of the springs are rusted out the pin will move in and out randomly causing stumbling

[marlinh]

Does it stumble in cruise going a constant speed, and at what speed and gear? Or climbing a grade? Engine cold or fully warmed up? Sometimes extra info will help pinpoint problems easier.

[sbc1116]

Does it stumble in cruise going a constant speed, and at what speed and gear? Or climbing a grade? Engine cold or fully warmed up? Sometimes extra info will help pinpoint problems easier.

I don't have cruise but I do notice it more on the highway at 70mph. Engine is warm, running in 5th gear.
I put 125 miles on it on Sunday driving 50-65 on a 2 lane mtn road and it ran fine. Yesterday coming home from work running on a 4 lane freeway bucking a wind than it acted up. It acts up when you are trying to maintain a constant freeway speed.

[dlb]

have you checked the things i mentioned in my last post?

[Petros]

sure sounds like a partially plugged fuel filer. if the filter is good could perhaps be a pin hole leak in the fuel line a head of the fuel pump (causing it to suck air). Possibly the screen in the fuel take pick-up screen has junk stuck to it, partially blocking the fuel inlet (that can be inspected though the hatch under the carpet in the back, use a flash light). Usually this will not cause poor economy.

Might also be a marginal coil causing weak spark when there is a high speed demand, but works okay at lower speeds and loads. check for a strong blueish-white spark that goes "snap-snap-snap!" when you hold a spark plug wire next to metal. If weak or yellow you have something wrong with the coil, inginor or distributor. That will cause poor economy too.

Put a timing light on it and see if the spark holds a steady timing, and the advance is working when you reve the engine. If the distributor backer plate or shaft is sloppy or loose it will cause unsteady timing and poor economy.

good luck.

[sbc1116]

have you checked the things i mentioned in my last post?

No I haven't had a chance to get back to it yet.
I work an 8-5 job Monday thru Friday and have a home business so I am limited to weekends for working on the car.
Thankfully it isn't my only form of transportation.

Thanks for all the help from everyone. It gives me some directions to go.

[marlinh]

I didn't mean cruise control, just driving down the highway at a constant speed.

[sbc1116]

I didn't mean cruise control, just driving down the highway at a constant speed.

Yes that is when I notice it. Highway speed trying to maintain a constant speed