Hello Everyone,

This is my first post and I hope that I can get some genuine help, because I am stumped. Some quick background on me, I have been playing with cars since my teenage years. I am not a professional mechanic, but I consider myself above average in terms of my wrenching abilities. I have built several hot rods from the ground up, but my daily driver truck is kicking my butt.

I have owned this truck since it had 28K miles. I am the second owner and I knew the first owner personally. Since I have owned it, I have taken good care of it, etc...etc...etc...It now has 108K miles.

A while back I started having the dreaded "rough idle" issue. To make a long story short, this lead to a head replacement due to recessed valves. The head replacement solved the rough idle problem, but now I have a new (did not have this issue before) problem.

ISSUE: There is an occasional, but persistent stumble when taking off. It tends to occur more if the engine is lugged down. It cuts out, almost like a momentary ignition cut. I can usually catch it if I press the clutch in, but occasionally it will stall.

The stumble also occurs under when accelerating and also while cruising at a constant speed. The whole truck will buck and jerk, as if the ignition is missing.

There is also a stumble when the throttle is "blipped" off idle with the truck not moving. It acts much like a lean stumble on a carbureted engine.

Items replaced in the last 2 month:

IAC
EGR valve and tube
Head
Timing belt and tensioner
Plugs, wires, coils
Crank sensor
Water pump
Upper water neck
Both coolant temp sensors
Accessory drive belt
Thermostat
Upper and lower radiator hoses
Air filter

I have also verified fuel pressure at the rail (65psi). The MAF has been cleaned with proper cleaner as has the throttle body. There are no codes.

When we replaced the head we paid very careful attention to the cam timing marks. We lined up the cam with the triangles, the oil pump with the diamonds and the crank with the dot. Once the tensioner was set, we rotated the engine two revolutions and confirmed that all the marks were still lined up. Once it was assembled, we reconfirmed the diamonds and the triangles through the inspection plugs with the balancer timing mark at TDC.

I know this truck has a lot more life in it, but the recent repairs have me frustrated.

Any and all help is greatly appreciated!

Andrew

 

Welcome to the forum

The electronics "they" add to an engine makes it more reliable, but its still a gasoline engine so your past work with carbs and distributors will come in handy.

Don't let the electronics throw you, troubleshooting is the same as it has always been
Fuel
Spark
Compression

Your description of "lean" condition on blipping the throttle could point to the problem

What caused that on a carb engine?
Failing Accelerator pump, it's job was to squirt in extra fuel to get instant throttle response and allow the Jets to catch up to the change in airflow.

So what does that on a fuel injected engine?
MAF sensor reads air flow, but can't react that fast, so what else?

TPS(throttle position sensor), whats it purpose?
Yes, it acts as the accelerator pump, gives computer instant feedback on throttle position, so computer gets the "heads up" instantly to add more fuel quickly.

TPS is easy to test
It has 3 wires
top wire will get 5volts from the computer as Reference voltage
center wire is the Signal voltage that tells computer throttle position
Lower wire is Ground

You can tap center wire using a sewing needle
Ground volt meter on engine
Key on engine off
center wire should be under 1 volt, .69 to .99 is spec
As you manually open throttle plate voltage should go up
At WOT it should be 4.5 to 4.9volts

Voltage should react instantly to changes in throttle position, and be steady at any fixed position, no quick jumps or drops as you move throttle thru its range of motion
One of the benefits of fuel injection is that 0 gasoline is used when decelerating, i.e. when you take your foot off the gas pedal computer turns off fuel injectors until RPMs are down to about 1,200.
If TPS voltage suddenly drops, injectors are OFF, stumbling follows if you didn't actually take your foot off the gas pedal.
And a heads up here, with fuel injection never coast down a hill engine idling, take your foot off the gas pedal and leave transmission in gear, 0 gasoline being used now, vs idle use

TPS is basically like a light dimmer or volume control, a variable resistor.


Simple test for vacuum leaks
Fuel injected engines can't use idle screws, so they all use an IAC(idle air control) valve, that's what Ford calls theirs.

It will be on the upper intake by throttle plate, it controls air flow thru a passage that bypasses the throttle plate
It is a 12volt solenoid with a spring holding it's valve closed
It gets 12volts with key on
Computer controls its Ground wire, it pulses the ground to vary the voltage, this allows very precise control of idle RPMs, +/- 5rpm.

After engine is warmed up, and idling, unplug the IAC Valve's connector, it will close
RPMs should drop to about 500, or engine may even stall, either is good, it means no vacuum leaks.

If idle stays the same then there is a leak, leave IAC Valve unplugged and start pulling hoses and blocking that port, to see if RPMs drop
PCV hose is most common vacuum leak, or PCV valve itself.


Vacuum gauge is still one of the better tools to diagnose, what is essentially a self powered air pump, they are only $20 and can tell you alot.
18-21" vacuum means valve timing is good
You can open the throttle and get instant drop of vacuum then close it and get higher than "normal" vacuum as RPMs drop, if its slow to react the exhaust could be clogged


Also on these Dual spark plug, dual coil pack engines you can test the coils and spark plugs by unplugging the 3 wire connector on a coil pack and see if the engine runs OK, RPMs will be slightly lower but there should be no misfires.

Each spark plug in each cylinder fires at the same time, there is no alternating, engine can/should run with only 1 coil pack and those 4 spark plugs working
Engine runs better with both spark plugs working

 

Ron,

Thanks for the detailed description. I am actually quite versed with EFI. In fact, I have not had a carb on my personal hot rods since 1991.

My latest couple of builds included LS swaps.

The IAC is new, but I suppose that doesn't mean it is good. I will do that test.

I have read about the TPS and will test it with the procedure you described.

I think I have a vacuum gauge banging around somewhere. Thanks for the 18-21" target base line numbers.

Andrew

 

I find this article a good guide for testing any gasoline engine with vacuum gauge: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

I don't think IAC Valve is the problem, just useful to disconnect for vacuum leak test

MAF sensor is the most common cause for bogging/stumbling, I didn't mention it because you seemed to know that by talking about cleaning it.
It uses similar voltage as TPS
1995 and up Rangers used 6 wire MAF
The 2 outside wires are for Air Temp(IAT sensor)
The 2 inside wires are for MAF sensor
The wires in between are the 12v/ground for the heated wire

So you can test MAF sensor, like the TPS, using the sewing needle and volt meter
Engine running, find which of the two middle wires has the lower voltage, usually about .6v
Raise RPMs and voltage will go up with air flow, keep a steady RPM and make sure voltage is steady
If there is a stumble try to determine if MAF voltage changed and THEN the stumble, or the stumble first and THEN the voltage changed.
i.e. is the MAF voltage causing the stumble or is the voltage change a reaction to the stumble

 

I went ahead and changed the MAF. When I took the old one off, there was some distinct discoloration on one of the two elements. Also, the MAF is 4 wire and there is a separate AIT sensor in the intake tube.

The new MAF made things way better, especially off idle, when starting from the line. It also fixed the idle when the AC was turned on. Before, when the AC was on, the idle would start to hunt and oscillate between 500 & 800 rpm. That's gone. Rock steady idle.

However, on my drive home, I went up a hill, put it in 5th gear. With the RPM around 2000 and 50-75% throttle, it started to buck.

I'm open to further suggestions.

Andrew

 

My old truck did just what yours is doing and it was o2 sensor just a thought

 

I appreciate the input. I will probably replace it. As far as I know, it is original, so it's time. Also, because I've just about replaced everything else, so why not?!

Gas mileage is terrible, so that's another sign that the O2 is going.

Andrew

 

When going up the hill what happened when you push gas pedal to the floor, WOT(wide open throttle)?

At WOT computer doesn't use O2 sensor, economy is out the window, lol, computer just feeds lots of fuel into the engine.
WOT is a diagnostic tool, although it probably can't get you out of a speeding ticket

 

I honestly can't say what it does going up a hill at WOT. I feel like it didn't matter whether it was moderate throttle or WOT, but I can't say that for sure. Next time I drive it, I will pay more attention and do more experimenting. No risk of a speeding ticket, this thing is slow.

To be clear, I think what you mean is that under a WOT condition, the computer goes into open loop and doesn't take feedback from the O2 sensor. I guess I sort of knew that, but I forget for relatively simple older ECUs were.

On my Cougar I have Holley EFI and it uses dual wideband O2 sensors, and it runs closed loop under WOT.

Thanks for all the great ideas! Please keep them coming.

Andrew

 

It was actually the change to Wide Band O2 sensors(0v-5v) that allowed Closed Loop at WOT

Rangers all used narrow band O2s, at least until 2009, probably '10-'12 as well

 

Did you ever find a resolve for this? I am having a similar issue. Tends to happen under load and up hills, about 1500-2k rpms. I have the 2.5L. i also come from the carburetor works so this threads been incredibly helpful. I’ll be running the tps test and stuff later today.

Usually mine happens just after you’re completely off the clutch and just start going a couple feet, then boom very jarring, more so than you’re typical fuel or ignition cut. But I’ve had it happen doing 30-40mph in 2nd or 3rd too.

There’s a deep audible single metal thud that happens when mine does it.

I have been able to replicate it once with the truck in neutral and standing still and using my thumb to blip the throttle under the hood. I noticed when it did this the entire engine threw itself to the passenger side. Violently. When it does this I feel it in the shifter and everything. The motor mounts are fine though.

The only code I have is for EGR, and I have a sizable exhaust leak somewhere around the cat. I’m hoping the exhaust leak/EGR is the culprit… I’m surprised I don’t have any 02 codes. My plugs look lean, they’re just as clean as when I put them in 100 miles ago.

 

Triple check the firing order on your coil packs

Then unplug either coil pack's 3 wire connector, so only 1 coil pack is working, and go for a drive
Should run normally no misfires, this Lima SOHC engine ran on just 4 spark plugs from 1974 thru 1988
Then switch it around, testing the other coil pack and its 4 spark plugs

Should do this once a year just to make sure all 4 spark plugs on each side are working
With dual plug engines you can't tell if one spark plug fails

Your description of "violent" stumble reads like a cylinder is firing right at TDC or just before TDC, instead of after TDC or just a simple misfire and stumble

 

Everything you state right there, indicates at least one broken motor mount.