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Harmony162 , 3 Minutes Ago
Member

moved this post since I had put it in the wrong thread working on a iPhone these buttons are so small and everybody seems to be so sensitive about that.

97 Ranger 3.0 Auto. Have owned the truck about a year and a half no real problems except the usual vacuum hose and what have you but lately I have developed a bit of a surge right off idle I would say around 12 to 1400 RPM just a small surge. When drop it in drive and push down a little bit and she will pull off and then try to dip down a 100 rpm or so then pick back up dip down and after two or three times it will settle down and run fine. Now here’s the kicker it only does this when the outside temperature is between 40° and 50° below 40° It doesn’t do it above 50° it doesn’t do it. Doesn’t seem to matter whether it’s morning or evening or rain or shine only if the engine has sat for hours and cooled down. The only modifications that I have made to the intake or exhaust system is I have plugged the 3/16 hole in the throttle plate. Did that about a year ago trying to fix a bad surge which turned out to be a leaky brake booster diaphragm. I just never went back and took the screw out of the throttle plate. Hoping there is an expert on here that can tell me where to start the search or possibly someone who has had the same problem and found out how to fix it. I have never had a code so no help there. All replies will be appreciated and in advance many thanks, Larry

 

By, "just a small small surge", do you mean in time duration?
I don't know how these engines normalize but there might be a vacuum leak. PCV and associated plastic piping OK?
Can the 40º to 50º effect range point to a bad CTS?
Maybe your 25 year old (worn) O2 sensors are sending crazy signals in a warmed system?
Can the weight of a 3/16" screw in a throttle plate (in a worn TB) effect its swing?

Dose it well with Techron, change the fuel filter and clean the TB & IAC valve as part of your inspection.

 

Hadn’t thought about the O2 sensors either, how do you check them?
Larry

 

A surge in RPM usually means air flow change, but can be change an air/fuel mix change from computer
Air/fuel mix is controlled by the PCM(computer)
PCM uses several sensors to adjust the air/fuel mix "on the fly"

Because you say it seems to be temperature dependent start with temp sensors
IAT(ACT) sensor tells PCM the air temp of the incoming air, cold air is heavier than warm air so needs more fuel added

ECT(engine coolant temp) sensor, tells computer how much CHOKE to set on cold start, all gasoline engines need to be choked until coolant temp reaches 160degF, along with the Rich fuel mix(Choke) a higher idle is needed, 1,200rpm at coolant temp below 70deg, 1,000 at 130deg, 800 above 160deg(these target RPMs change by year and software)

MAF(mass air flow) sensor, not really an "air flow sensor", it tells computer the WEIGHT of the incoming air, as said cold air is heavier than warm air, computer is already programmed for a 3 LITER engine(3.0l), so it knows EXACTLY how much air is "flowing in" at any RPM, its just math, what it doesn't know is the WEIGHT of that air, which is temperature based, and needed since the 14.7:1 air:fuel mix is WEIGHT Based, 14.7 pounds or air to 1 pound of gasoline

So those are the 3 temp sensors the computer uses

Then there is the idle control itself, the IAC(idle air control) Valve, PCM uses this to set idle RPMs and its accurate +/- 4rpm so very accurate
Surge in idle or just off idle could be a failing IAC Valve, it has a small opening, valve only moves 3/8" from closed to fully open, and thats 500 to 2,000rpm range, so even slight movement of the valve can cause a drop or surge in RPM and then PCM has to react to that sending IAC Valve more or less voltage to restore position, which you "see" as a surge

TPS(throttle position sensor), this sensor gives the PCM a "heads up" that driver wants to accelerate, when throttle opens there is a sudden in rush of air, the engine would stumble/hesitate unless the PCM injected extra fuel instantly ahead of the increase in RPM, if you had a dead spot on the TPS it could be "telling" PCM that you pushed down on gas pedal and then let off gas pedal(dead spot), then pushed it down again


O2 sensor shouldn't cause your symptom, but they do need to be changed every 125k miles or 12 years, they run out of chemicals, same as battery do they just last longer
MPG goes down, so they pay for themselves over the 125k miles in fuel savings

 

RonD: from your description of everything I believe the problem is going to be in either the IAT(ACT, the IAC or the tps. Not sure but I believe I replaced the iac when I was hunting the large surging idle which turned out to be the brake booster. (Remember that one) The engine doesn’t try to stall it speeds up when I first press the accelerator and then it will drop down and pick back up & drop down then pick that up. Usually only does it three or four times and then it will pull smoothly at the desired rpm. invoices I received from the PO indicate the O2 sensors were replaced just before I got the truck. The tail pipe and a couple of spark plugs I looked at are not black but they are charcoal not gray as you would want them to be. Also fuel mileage is a little less than it should be. But then I really drive it more than 10 miles at a time
Larry H

 

Clean the MAF sensor, always, when there is any stumbling or hesitation, cheap fix

You can test the TPS with volt meter and sewing pin
Put pin into the center wire on TPS, this is the return signal the PCM sees
Set volt meter to DC volts
Ground the meter to battery or engine metal
Turn key on, engine off
Test voltage on center wire, 0.6 to 0.9volt is expected
Slowly open throttle and watch voltage, should slowly go up, no jumps or drops
At wide open it should be 4.5volts to 4.8volts
Then close throttle and repeat a few times

The TPS is a "learned" sensor, so voltage is not specific, this allows for variations in new parts
But it needs to be steady as it goes up and down, no jumping or dropping

The IAT or ECT sensors you just change if in doubt, not expensive parts, you can unplug IAT and drive that way, you will get a code but thats GOOD it means PCM was using that sensor and see if the surge goes away with no input of that sensor

 

Got to thinking while installing the new 2 row radiator a few weeks back I had the IAT in and out several times that’s the first thing I’m going to replace. It appears to be the original. I was wondering Ron, do you reckon a Dodge ram IAT would be compatible or are they very specific to the vehicle system? I have a new IAT that I bought for my 04 ram 5.7. It looks the same I am sure it will fit the intake tube. Haven’t checked Wiring too cold outside right now. The reason I’m asking is because those suckers are 30 bucks.
finally, what is the other sensor located on the intake tube Just before it goes in to the air cleaner?

Larry H

 

Might work, the Ford sensors are 5volt, so certainly would hurt if Dodge was 12v

You can test both to see what OHMs are at room temp then heat each one up a bit and see if they match doesn't need to be exact just close