RPM's up&down

[werewolf]

my 1987 ranger 4x4 has been driving nuts. it's been idling rough, i did a tune up plugs and wires. still idle rough. I got lucky and found a green vac line broken to the bottom of throttle body. repaired it, and the truck runs a lot better. But now when you start it up cold it try's to die at first and then it idles up and down by it's self for about a minute and then it idles just fine. So do anyone have a ideal on why for a minute it's up and down. thanks for and help

 

[RonD]

What engine is it?


Could try cleaning the IAC(idle air control) Valve.

Fuel injected engines can't use an idle screw or a Choke Plate.
So computer needs to know when engine is cold and when it is warm, it does that using a temp SENSOR, ECT sensor, this is a 2 wire temp device, not to be confused with the 1 wire SENDER used by the dash board temp gauge.

So when engine is cold the computer raise the idle to 1,100rpm(approx.) and adds extra fuel(like a choke plate did).
As engine warms up idle starts to go down until it gets to warm idle, 625rpm for manual trans, 750 for auto, it sets these idles using the IAC Valve.

IAC Valve is an electric motor, a step or stepper motor, connected to a valve that is on an air passage that bypasses the throttle plate.
If valve is fully open RPMs could get up to 2,000, fully closed should be 500rpms.
One thing you can try when engine is fully warmed up and idling, unplug the wires on the IAC Valve, it will close all the way, RPMs should drop to 500 or engine may even stall, either is OK, it means no more vacuum leaks.
If idle stays up, 700+, then you do have a vacuum leak.

IAC Valve can get dirty inside, so valve gets sticky, computer tries to open and close it to set idle at 1,000(for example) but first try gets 800 so it opens it more and it unsticks and idle is now 1,200, so it closes it more, then 900, and so on....... because where the valve needs to be in the tube is sticky so computer can't get it right.
After warm up, where valve sits most of the time all is well.

Not hard to clean, couple of bolts and maybe a gasket if it can't be reused, usually they can be reused

 

[werewolf]

RPM's

Thanks for the info on this. I will for sure run this test and see what i get. this gives me some where to go with this. Thanks again. sorry engine is 2.9 v6, manual 5 speed

 

[werewolf]

Sorry about that

The engine is 2.9 v6. manual 5 speed trans.

 

[enjr44]

Probably IAC. But, check the connections to the ETC, the two wire one. Maybe you messed up the connections doing the tuneup.

 

[werewolf]

RPM's

first of all, thanks to RonD. I pulled off my IAC valve and found that someone has had it off before. and they must have missed up the gasket and used red silicon. I cleaned out the silicon and took B-12 and cleaned the valve. you can move the plunger freely. I made a gasket for it. Now when i start it cold it idles up to about 1500, rpm and drops down to 400 rpm and dies. it does this two times and the third time i start it it will idle at 1200 rpm till it warms up and it drops to around 700 rpm. And i unplugged the iAC vale and the rpm's drops to around 400,to 500 rpm. you think i still have a vac. leak or need to replace the IAC valve, I sprayed B-12 all around the top of the engine. the rpm stayed the same. thanks.

 

[RonD]

Now reads like computer can't read Coolant temp(engine temp) accurately.

Computer relies on the ECT(engine coolant temp) sensor to set cold idle and warm idle RPMs.
This is a 2 wire sensor only used by the computer, there is a similar looking SENDER, with only 1 wire, that is used only by dashboard gauge.
Both are usually located near thermostat housing.

When powered up the computer sends the ECT sensor 5 volts on one wire, ECT is a resistance sensor, when cold resistance is low.
If cold the computer should get back above 3volts on the other wire.
As engine warms up that voltage starts to drop, when fully warmed up, 185degF, this voltage will be under 1 volts, .5v is spec.

When you start a fuel injected engine the computer will always open the IAC valve all the way, so high RPMs on startup are normal cold or warm.
Computer then checks ECT temp to set idle for cold or warm engine, high if cold normal if warm.

If engine was cold but voltage from ECT is low then computer would close IAC Valve too much(700rpm idle) and the cold engine would stumble and stall because a cold engine needs more fuel and higher RPM(IAC open more) to stay running, cold engine needs to be Choked like a carb engine was.
That is another function of the ECT sensor, computer runs a Choke Mode for cold engine, Richer fuel mix, higher idle and advanced spark timing.
Without this a cold engine can't run or run very well.


Because you say it does seem to work after 2nd or 3rd try it could just be a loose connection at ECT.
You can test voltage on both wires using a sewing pin to pierce the wire.
One will have 4.8-5.0volts, this is the power from computer.
Other will have under 4 volts, 3volts approx. on cold engine.

 

[werewolf]

temp

I'm bad, did not think of that. I just ordered a new radiator this morning. right hand tank started leaking when i was doing the tune up. found one on ebay. I'm going to replace thermostat and the sensor while i wait on the radiator. And thanks RonD.

 

[RonD]

Thanks

Be sure to use a 192 or 195 thermostat, with jiggle valve.

The higher temp means oil stays cleaner and engine runs more efficiently, so win-win.

160 or 180 is NOT better for the engine, and can NOT prevent over heating, overheating is when coolant temps get above 230degF, any 160, 180, 195 or even 200degF t-stat would be open all the way at that point, so max cooling is in effect.
All a lower temp t-stat does do is make for dirtier oil and lower MPG

Jiggle valve is just a hole in the t-stat plate that allows air out when refilling cooling system, or if any air should get in it can be let out.
Jiggle part is the metal pin inside the hole that "jiggles" to keep hole open and clog free.

 

[werewolf]

good info

thanks, very good info. I will put it to use come Monday.

 

[werewolf]

RPM's

Thanks Mr. RonD for all the good info. I replaced the radiator, the thermostat and the housing, the old one bad shape on the inside. Replaced the two wire sensor. New rad. cap and top and bottom radiator hoses. It helped a lot, but still won't stay running on first fire up. Still dies one or twice,or if i keep the rpm's up to 1500 for a min. it will stay running. But i just reread your last post to me and i had forgot to check the voltage in and out of the the sensor. I'll make me a copy of your post and take with me to the shop Monday. Is there anything else i can check to try to get it to stay running at first start. And thanks again for your help. I'm getting closer every time.