A minor annoyance or something more serious

[DandysRanger]

The Ranger has been running great since I replaced the cylinder head. However it is doing something strange very consistently. It always starts right up but after a few miles it will start to misfire and buck a little. After 5-10 seconds it cleans up and keeps going. It does this 90% of the time warm or cold. The fuel pumps were replaced and the fuel filter is new. I've put in HEET for water in the fuel but I still get the same symptoms. Not sure if I should just ignore it or start tearing it apart again...

 

[Bird76Mojo]

Sounds like it could almost be something happening when the thermostat opens, it's then shoving coolant into a cylinder? You could check all of your spark plugs to look for signs of burning coolant. Which would lead you to a cracked head or a bad head gasket.

Also, edit your signature to include your year of Ranger, engine and transmission type, etc.. It helps save time and frustration when others are trying to help you.





GB

 

[DandysRanger]

Thanks Bird, signature edited.
It doesn't do it all the time and after the momentary stumble it runs as before, which is to say, excellent.

 

[Bird76Mojo]

That's a tough one to diagnose for sure. If the check engine light isn't coming on and it isn't throwing any codes.. Are you sure that the check engine light bulb in the dash is still good? It might not hurt to go to a local parts store and borrow their scanner real quick. Just to see if any codes are present.



GB

 

[tomw]

An 85 won't have a CEL. You have to pull the codes under hood under the plastic cover where the access connector flops around.
I had bad plug wires that looked to be in excellent condition. They gave me fits trying to figure out what was causing an intermittent bucking/jerking condition. Check for ohms, about 1k per foot(maybe IIRC). If infinite, look for replacements.
The EGR can also cause stumble. It is not supposed to have any vacuum below ~30mph(about), and if it leaks, will cause a cruddy idle or misfire. You can check by pushing on the atmosphere side of the diaphragm to open the valve and see that it works and closes properly.
Check the wires and see if they are damaged. I have no idea why they fail, and I never did anything to mine to kill them(pulling/flexing/etc). If nothing there, check temp sensors. Last thought is you are getting a hiccup when you go from 'open loop' to 'closed loop' when the computer/o2 sensor/injectors/temp all are at operating temperature, and the computer goes off fixed spark timing, and the O2 is used to fiddle with the fuel flow.
tom

 

[DandysRanger]

Thanks Tomw. I suspect the EGR (or lack thereof) may have something to do with it. The EGR port in the exhaust manifold is plugged and the EGR is disconnected from the harness. It probably does have something to do with going from open to closed loop since it is remarkably consistent.

I would like to hook the EGR back up again but the exhaust port is the wrong size so it will probably have to go to a machine shop to be drilled and rethreaded. I have a thread on that in this forum.

 

[RonD]

EGR is only used after engine is warmed up, so if symptoms are present when engine is cold then it isn't EGR related.

1985 still used MAP sensor not MAF sensor, I would check MAP sensor's vacuum hose and connection

MAP sensor is most likely on the firewall, small black plastic box, has a Vacuum hose attached and a 3 wire connector.
Check the vacuum hose, remove it, at both ends, and make sure its air tight, no cracks
Unplug the connector and make sure there is no corrosion, plug it back in.
Most sensor use very low voltage so any small bit of resistance in a connection can cause an issue, so the simple act of unplugging and plugging back in can restore a good connection.

MAP = Manifold absolute pressure, this sensor tells computer how to set air:fuel mix, so could cause bucking issue

TPS(throttle position sensor)
This sensor tells the computer when to add extra fuel for acceleration, it serves the same purpose as the Accelerator Pump did on a carb, so a hesitation in acceleration could be TPS issue
Not bucking while cruising, just when accelerating

 

[DandysRanger]

EGR is only used after engine is warmed up, so if symptoms are present when engine is cold then it isn't EGR related.

Hello RonD, It doesn't do it right off the bat. It is after it has been driven a few miles when the engine is warmed up some. Not all the time but pretty consistently. It will stutter some and then clean up and go as before. It does it only once per driving session.

I'll check the MAP as you suggested as well. Appreciate the help,
Jim

 

[RonD]

"It does this 90% of the time warm or cold."

So not this then, or are you referring to outside air temp not engine temp?

So it only does it AS engine warms up?

 

[DandysRanger]

No I am referring to engine temp.
So, like I stated before, it will start right up and run fine and then a few miles down the road I will experience the hiccup. It will pass and I will run into a store or whatever, come back out and it will start up fine and then a few miles down the road I get the same hiccup. It has more to do with the amount of time/miles after startup then anything else. Once I get thru the initial hiccup it won't do it again no matter how long I drive it.

 

[RonD]

When was the O2 sensor last changed, they are good for about 150k miles?

Fuel injection computers have two modes, Open Loop and Closed Loop
Open loop is when computer first boots up
Computer used air:fuel mix tables in memory to run the engine Rich, like carb did with Choke plate
Computer watches ECT(engine coolant temp) sensor to see when coolant gets above 140degF, computer also watches O2 sensor, doesn't use it, just watches it

O2 sensors can't work until they are heated above 600degF by the exhaust heat, with cold engine this takes a few minutes, and a minute or two even after a short shut down

When coolant gets above 140degF and O2 sensor seems to be switching voltage at least 6 times a second, computer will switch to Closed Loop
Closed Loop is when computer sets air:fuel mix on the fly based on O2 sensor feedback, i.e. is exhaust lean or rich
So in closed loop computer relies on MAP sensor, ECT sensor and O2 sensor to adjust air:fuel mix

It reads like this switch from Open Loop to Closed Loop is when you have the problem
Now it could be a computer issue, so nothing you do externally will correct it
But you could try changing O2 sensor if you don't know how old it is, O2s use a chemical reaction so they wear out, and changing them improves MPG over the next 150k miles so not a bad investment.

Changing ECT sensor is not expensive, this sensor is only used by the computer, there is an ECT SENDER also on the engine, and that is used only for the dash board temp gauge.

ECT sensor is 2 wire
ECT sender is 1 wire

Air:fuel mix is based on engine temp so if ECT sensor was showing 120degF(rich mix) and then suddenly jumped to 180degF(lean mix) engine would stumble
ECT also uses 5volt scale, 3volt cold, under 1volt warm, so any corrosion on the wiring/connector could cause similar issue

 

[DandysRanger]

Thanks RonD. I went thru my notes and I replaced the O2 sensor in 2012 which was about 20K miles ago. I have also replaced the ECT and ACT about the same time.
This began after I changed the cylinder head out and replaced the entire exhaust while deleting the EGR. Like I said it's more of an annoyance than anything else.
I'll take apart a few connectors and clean them with some DeOxit.
Jim

 

[RonD]

Did you disable or plug the EGR vacuum system?

If computer tries to open EGR valve that will be a vacuum leak at that time if its not disabled or hose is not plugged

There is an EGR solenoid with 2 wires and 2 vacuum hoses, you can unplug the wires so computer can't activate that solenoid
Follow the EGR valve's hose back to EGR solenoid

 

[DandysRanger]

I unplugged the EGR from the harness. I'll check the vacuum hoses.

 

[tomw]

If you have the EFI system(85 was more regional than national CA esp), it would have two solenoids under the plastic cover aft of the battery & starter relay. One feeds vacuum to the EGR, which moves and at the same time moves a POT, variable resistor, to change a resistance and thus voltage, fed back to the computer. The other solenoid is the 'dump' solenoid which opens to release the vacuum. FWIW.
Deletion may lead to spark knock and actually, poorer performance. It is only actuated after you have reached cruise speed and are not accelerating, engine at full temp. It should not interfere with any sort of burps or lurches as I think about it. You report the problem in the 'middle' of acceleration, not at tip in, and not at cruise. EGR would/should generally not be active at that time as I understand.
tom

Added: As I think about it, you could get a lurch or chug when the computer shifted from 'open loop' to 'closed loop' operation in the case where the 'closed loop' numbers were radically different from the 'open loop', assumed, from a recorded/fixed table of values that 'closed loop' uses. If the fuel metering or timing were so sar from open, it could take a few cycles for the computer to 'recover' and start responding to 'current conditions'. That could cause a 'lurch' when the values were initiated.
tom