98 4.0 rpm surge in gear when hot - Ranger-Forums - The Ultimate Ford Ranger Resource


4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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98 4.0 rpm surge in gear when hot

Hello, been reading posts for a week and can't seem to find one that helps me out. So, I joined and made one myself.
I just replaced my lower intake gasket and all other gaskets from there up. For the parts I took a part any way. If it wasn't necessary to separate it I didn't want to disturb it more than I already had. I replaced it because of a leaking gasket at the passenger side front seal for the head. Truck has 206,000 miles on it and ran great before I took it a part. It had plenty of power and started right up like it was new every time. After putting the truck back together and fuel returning it fired right up and was idling great. I thought wow that was too easy. I spoke too soon. As soon as it warmed up a little I put it in gear and started to surge, it does this evertime it's put in gear warm. It only surged a couple times and then stops but has almost no power and sounds really bad when you accelerate. Almost sounds like something grinding but no noise coming from the trans area and it sounds like grinding but not really it's definitely a different sound. I just want to add that the motor never overheated. Blew the gasket and I pulled over less than a mile after I saw the first small puff of steam. No signs of anti-freeze in the oil either.
My neighbor has nicer toys than me and let me use his snap-on scanner. Pending code was P0453. Memory codes are P0453, P0174, P1000, Ran the two tests that start with a K can't remember the names. One not running and one running. Not running I get the P0453. Running I get the P0453 and the P0505.

Right now I am not too worried about the P0453 if it's the Evap system. The Haynes book says its a Fuel pressure sensor circuit, low input. The Snap-on scanner says it's a Evap system issue. Which one do I believe??

Saturday I am going to change all the vacuum lines and put the new o-ring on the EGR tube, I didn't see it in the kit at first. I am hoping this will resolve all the issues except the P0453 but maybe that too. Any thoughts?
If no change I am going to replace the IAC valve ($50.99 AutoZone) because of the P0505 code.
If no change I am kind of back at square one. I expect issues when I take things apart but really didn't expect it with a simple gasket swap on a motor that ran great.

If anyone has a link to a diagram of the vacuum lines that would be helpful in pointing out any I can't really see.
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Old 1 Week Ago
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Welcome to the forum

4.0l OHV engine was used in Rangers from 1990-2000, in 2001 Rangers got the 4.0l SOHC engine, no relation to the 4.0l OHV engine

I don't think you can technically "blow" an intake gasket unless you lose an intake valve or get a MAJOR backfire , lol.
Intake is always negative air pressure when engine is running
So you could "suck" an intake gasket, although I have never read that term used, lol

The 4.0l OHV had an aluminum intake bolted to cast iron heads, bad combo as the metal types cause electrolysis where the coolant flows thru the intake from one head to the other.
And the gaskets at the corners of lower intake can "blow" from coolant pressure after warm up, so I stand corrected, intake gasket can "blow" at the corners.

Vacuum diagram for your model should be on the rad support
Vacuum systems varied by State emissions requirements so they had to put label on each vehicle
You most likely have Federal Emissions vacuum system in Illinois

But first, lets see if you even have a vacuum issue
On the upper intake on throttle body is the IAC(idle air control) Valve
Looks like a can on its side and has a 2 wire connector.

After engine is warmed up, and idling, unplug IAC's 2 wire connector
IAC Valve will close and RPMs should drop to about 500, engine may even stall, either is GOOD, it means no vacuum leaks.

If RPMs do not change then there is a vacuum leak
But look at one more thing, on the throttle linkage there will be what "looks like" an idle screw, it isn't, turn it counter clockwise while IAC valve is still unplugged
If RPMs start to drop keep turning screw until engine is at 500 or almost stalling, leave it there
It is an anti-diesel screw, fuel injection can't use an idle screw, no Jets, but some people adjust this screw without knowing how to do it or what its for.

If turning screw doesn't change RPMs then turn it back to where it was and leave it there, check for where vacuum leak is, start at larger hoses on intake

I assume an automatic trans from the comment, "put it in gear"
When you shift into Reverse or Drive the computer will open the IAC Valve a bit more to increase engine RPMs because of engine load "in gear".
Assuming warmed up engine, if Park idle is 700 then "in gear" idle will be 750, so about a 50rpm bump up
Turning on AC does the same thing, computer bumps up idle rpm to compensate for load


Computer gets RPM data from crank sensor, and it uses IAC Valve to set "target" idle RPMs, factory programmed RPMs, based on coolant temp
Computer is accurate within +/-5rpm, on my 4.0l ohv, manual trans, target is 625rpm, and it idles between 622-628, so very very responsive and accurate.


IAC valves do get dirty, and sticky.
Lets say when you shift into gear the computer bumps up the idle but IAC Valve sticks, so computer tries to open it more and more and then it "unsticks" and RPMs jump up, so computer must now close it to get Target RPMs and it's sticky so rpms drop below Target.....
So you get up and down idle when you put it in gear, surges in RPM, until computer can stabilize IAC Valve position

I would clean the IAC Valve first, good read here on that: How to: IAC valve cleaning thread (w/pics) | Ford Explorer and Ford Ranger Forums - Serious Explorations


TPS(throttle position sensor) allows driver control of computers air:fuel mix
When throttle is closed TPS is sending computer under 1volt, .69-.99volts is spec
At this time computer is running idle's air:fuel mix
Above 1 volt the computer is taking input from drivers foot on gas pedal.
Range is 1volt to 4.5volts

TPS is a variable resistor, like a light dimmer or volume control, it gets 5volts and then sends back .69 to 4.5volts depending on throttle position
It can wear out, not often but not unheard of either.
If TPS was sending back an unstable voltage, say it was jumping from 1.1 to 1.5volts and back, computer would "think" drivers is tapping the throttle, so rpms could wander up and down.
Very easy to test this
Center wire on TPS is the .69 to 4.5volts
Get a sewing pin/needle, and pierce that wire
Set Volt meter to DC Volts
Turn on the key, engine off
Ground meter
Test voltage on center wire, should be .69-.99, under 1 volt
Now manually open throttle and watch the voltage, should be nice and stable, increasing as throttle opens
At WOT(wide open throttle) it should be 4.5 to 4.9volts
Any jumping around or drops in voltage means replace TPS

Adjusting that anti-diesel screw also changes TPS voltage as it changes the throttle plate position, so.............NOT an idle screw, lol.


Yes, EVAP code would not be related to RPM issues
EVAP system seals the gas tank and then puts negative pressure in the tank to keep gas fumes from getting out, pollution, while driving around, while gas sloshes around the tank more fumes are created.
When you stop to fill up the tank you should hear the "whooshing" noise when you open the gas cap, thats from EVAP system and the negative pressure
If you haven't heard that in awhile then EVAP system does have a leak
The negative pressure doesn't stay in the tank after key is off, after a few hours it will be equalized, but when stopping for gas you should hear the "whoosh"

EVAP has a pressure sensor, not fuel pressure, negative air pressure
EVAP controls a solenoid on a vacuum line, computer opens that solenoid once engine is running and then watches for changes in the pressure sensor so it knows vacuum is lowering air pressure in the tank.
Then it closes solenoid and watches pressure sensor to make sure pressure remains in the tank, no leaks, if pressure doesn't hold you get the "Check gas cap" light or code

When looking up any DTC code look at ALL the codes for that system, they are grouped together
The computer is not all that smart, but it chooses one code over another for a reason, so the codes the computer did NOT choose will help you eliminate things to check

Last edited by RonD; 1 Week Ago at 01:11 PM.
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Thank you for such a lengthy reply. The gasket blew on the coolant side, passenger side front.
I did a lot better than I thought putting it back together but after I finished I re-routed the spark plug wires because I didn't like how I had them and in the process crossed 5 and 6. After a good night sleep I saw it right away. I am usually pretty good at getting my stuff together right but I slipped up on this one. Runs great like nothing ever happened. I didn't even question the wires because I checked a few times before I re-routed them. It was idling surprisingly smooth for having two cylinders firing wrong too. Only showed a sign of trouble when put in gear.
Thank you for all you input though. I will be addressing the rest of the codes now that I am back on the road and the IAC is first on the list. I did unplug it the other day and the truck died. I see by your comment that I was right. When it died after unhooking the wire it was a good thing. I assumed it meant the valve was working.
Again thanks for the time and the information.
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