Cold start valve?

[Ralph Mills]

I have a 94 4.0 and it has been bogging down lately. It will run fine for a while then bogged down for a while then run fine Again. At start up when the engine is cold it runs very well but as soon as it starts to warm up is when the bogging down starts to happen.
I was told by a mechanic that the problem lies with the cold start valve. I have not been able to find any information whatsoever on a part called a cold start valve for the 1994 4.0 motor. So far all that I have done was to ensure that the MAF was clean.
Does anyone know what the cold start valve is? Is it part of the fuel injectors system?

 

[RonD]

Welcome to TRS

No cold start valve on Rangers

It is a separate fuel injector that adds extra fuel when an engine is cold, like what a Choke Plate did for a Carb engine.

Ford does the extra fuel using the regular fuel injectors and an engine temp sensor, engine computer runs a Choke Mode, part of Open Loop operation in the computer software.
Choke mode is Rich fuel mix, high idle and advanced spark timing.

Does your engine idle at 1,100rpm or so on cold start, it should, that means Choke mode is set.
As engine warms up idle should slowly drop, at full warm up, 5 to 8 minutes, you should have 650rpm idle with manual trans, or 750 automatic trans.
If it is doing this then computer, IAC Valve, and ECT(engine coolant temp) sensor are all working.

After that 5 to 8 minutes the computer switches over to Closed Loop and uses the O2 sensors and MAF sensor, and that seems to be when you have the problem.

4.0l OHV engine would bog down when MAF was dirty, common issue.

What's your MPG like recently?
You could have some extra fuel leaking in from the Fuel Pressure Regulator(FPR).
It is top front of engine behind alternator
It has the larger Fuel Return hose attached and a smaller Vacuum hose, locate the vacuum hose and pull it off to see if there is gasoline in that hose, there shouldn't be
If so then you will need to replace FPR.

After engine is warmed up and idling, located the IAC Valve, upper intake drivers side, looks like a can on its side.
IAC Valve has 2 wire connector, unplug it while engine is warm and idling
RPMs should drop to 500 or so or engine may even stall, either is good it means no vacuum leaks

If RPMs do not drop then you do have a vacuum leak and that can cause bogging

 

[Ralph Mills]

It is a manual transmission and idles at 800 RPMs after warm up. I have cleaned the mass air flow sensor. One thing I have noticed is that even after driving the vehicle for 30 to 40 minutes the temperature gauge does not rise much further than just passed the first line. I am getting plenty of hot air when I turn on the heat so I assumed that the temperature sensor and sender were bad so I replaced both. There still is no change in the temperature reading after replacing them. I have not replaced the thermostat yet. Could a faulty low temperature reading be the cause of the wrong ratio of air to gas mixture which would cause the bogging down that I experienced?
I am also going to check The other possible issues that you mentioned.
Thanks!

 

[RonD]

Ford temp gauge uses 210degF as 1/2 way point, lines and letters on the gauge aren't helpful since they changed every few years.
1/4, 1/3, 1/2, 2/3, 3/4 are best way to describe needle location on temp gauge.

Ford switched to 190-195degF thermostats in the late 1970's, early 1980s, after S.A.E.(and Ford) research showed, operating temps of 190-195degF got better MPG and cleaned the oil(burned off contaminants) better than 180degF operating temps(thermostats)

So your temp needle should be just below 1/2, and above 1/3, after full warm up.
Yes, could be thermostat is opening too much or stuck open(failsafe mode), OR previous owner put in a 160-180degF thermostat.
That would explain the 800rpm idle on manual trans, should be 625-650rpm after full warm up on 1994.
Computer bases idle RPM on coolant temp.

Easy test for stuck open thermostat is to start COLD engine
Open the hood and feel upper rad hose, at radiator end
Let engine idle for 2 or 3 minutes then feel either heater hose and then the upper rad hose.
Heater hoses should be warm, upper rad hose should still be stone cold, if upper rad hose is warming up then thermostat is open and should be closed.
Thermostat shouldn't allow warm coolant to flow to radiator until thermostat is warmed up to 190degF, and that usually takes 5 to 8minutes depending on outside temp

 

[Ralph Mills]

That makes sense! I just started it and checked it and after only about 2 minutes the top radiator hose was just as hot as the heater hoses. So hopefully putting in a new thermostat should correct that problem. Thank you for all that information!

 

[RonD]

Most thermostats have a "jiggle valve" and yes that is the technical term, lol, go figure

It is a small hole in the thermostat plate with a loose metal pin in it.
When installing a side mounted thermostat(which 4.0l has), place the jiggle valve at the 12:00 position.
It is there to let air out while refilling the coolant or if air gets in from a leak, it is purged out that hole during warmup because jiggle valve is at 12:00, high point.

The metal pin is there so hole doesn't get blocked/clogged, coolant flow "jiggles" the pin and keeps hole open.

If new thermostat doesn't have a jiggle valve then you can drill a 1/8" hole in the plate and install it at 12:00, same as jiggle valve without the pin

When refilling coolant in pretty much any engine, pull off 1 heater hose from the firewall, then fill system via radiator until coolant starts to flow from heater core AND it's hose, then reconnect them, any remaining air won't be much and will self purge into top of radiator and then Overflow tank after a few warmups and cool downs

 

[Ralph Mills]

Replaced thermostat and temp is where it should be. Bogging down still happens mostly after engine warms up still. I unpugged MAF and it runs better consistently but obviously still not perfect and engine will cut off after stopping at red light but starts again just fine. Engine will also cut off if idling and then unplug MAF.

 

[RonD]

Does your engine idle at about 1,100rpm on Cold Start?

When you start it cold, without touching gas pedal, does engine REV up to 1,500rpm then drop down to 1,100?

 

[Ralph Mills]

Yes sir right at 1100 until it warms up.

 

[Ralph Mills]

Dont remeber seeing it hit 1500 but Ill let it cool down and check.

 

[RonD]

REV up is not a big deal, just making sure computer, ECT and IAC valve are all working together

 

[Ralph Mills]

Cold start without MAF plugged in with rpm's at 1500 for a couple of seconds then down to 1100.

 

[RonD]

Unplug TPS(throttle position sensor), and plug MAF back in and see if it starts OK

 

[Ralph Mills]

I did and it started just fine the exact same way. 1500 RPMs at first and then down to 1100.

 

[RonD]

So it started with MAF plugged in?

And it didn't start with MAF plugged in before?

So TPS decided that?