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90 ranger with 2.9 surging idle


FredrickPatton

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I have a 90 ranger with a brand new 2.9 in it and I cant seem to figure out why its surging at idle. Ive ran the computer and I got a code that says the coolant temp sensor is out of range and im not sure if that is telling the computer to act this way. Also not sure why the temp sensor isnt responding because it tested 3.74 ohms wich I thought was good unless im testing it wrong. Ive been over every possibility I can come up with. I cant find a vaccum leak, tps sensor tested good, iac tested good, new plugs wires cap and rotor. I didnt replace the harmonic balancer and the rubber ring has cracks in it so im not sure if that would cause it but I figured it would be shaking all the time if it was off and it doesnt so im not sure about that. Any ideas?
 


Andy D

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This is pure speculation The CTS open is cold and makes for richness . Open is the usual failure mode. Maybe yours shorted . I would looking for vacuum leaks and at the idle air valve,
 

RonD

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ECT sensor has two wires, it is the computers coolant temp sensor.
ECT sender has one wire , it is used only for the dash board temp gauge.
There are two coolant temp devices on all fuel injected vehicles, three if there is an e-fan :)

ECT sensor should be tested with volt meter.
Ground the meter, set to DC Voltage, with engine cold, key on, 1 wire will have 4.5-5volts, that's the "sensor reference voltage" from the computer, it is used for the TPS and IAT(air temp) sensors as well.
The other wire should be about 2.5-3volts engine cold, as engine warms up this voltage should drop to under 1volt when engine is fully warmed up.

As far as the idle:
With engine warmed up unplug IAC Valve, it should close all the way and idle should drop to about 500, it may stall out so you might have to prop open throttle slightly.
But without IAC Valve connected computer is not controlling idle so if surging stops, it was computer causing the surge.
If surging continues then recheck for vacuum leak with engine at this low idle.
MAF sensor is the start of the "vacuum" area of the engine, it isn't just the intake manifold, any air entering engine without passing thru MAF sensor is a "vacuum leak".

If you have a timing light use that as well to see if spark timing is jumping around causing the surging, use SPOUT connector for this
 
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FredrickPatton

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Ok thanks a lot guys I will try those ideas and get back to you!
 

FredrickPatton

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So first I would like to explain that I am not a mechanic so im just working off the little I know from my experience lol. So I realized the motor is not surging it is more of an intermittent miss. I did however unplug the iac valve and when the idle droped to 500 rpms the miss was still there. I also re checked for any vacuum leaks and could not find one. As I said before i put brand new plugs wires cap and rotor on the motor when I instaled it and it ran great at the time. I only have about 2,800 miles on it now. And again it is an intermittent miss, not all the time. Ive noticed the last couple days it misses at low rpms when driving but not so much at high rpms. Any ideas?
 

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I would pull out the spark plugs and keep track of which plug came from which cylinder.
Compare them side by side, one or two should be darker than the others, they are in the cylinders that are missing.

If you google: spark plug condition

You will see several photos of various spark plug tips and what causes the many colors.
If a spark plug looks super clean, i.e. steam cleaned, then you could have the start of a head gasket leak, hope not.

With engine warm, idling, and missing.
Get a timing light and test each spark plug wire, see if one wire is not sparking in regular rhythm.

Put some Seafoam in the gas tank to clean the fuel injectors, I do this once a year and it makes my 300k 4.0l idle much smoother.
 
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Spott

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Mine misses a bit like that, too. It's an 87 2.9. It's pretty strong while accelerating, but it seems to miss a bit when I'm cruising at a constant speed.
I'm fairly certain it's fouled fuel injectors, but I haven't been able to work on them yet. I intend to buy some rebuild kits like rockauto #SK57, and soaking them to get the shellac cleaned out.
 

FredrickPatton

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So I pulled the spark plugs out last week and didn't see much difference in them. I will do it again today though and Google the pics to see if I missing something. I really don't think its a head gasket leak considering the motor is brand new. I put fuel injector cleaner in a week ago and didn't seem to make a difference but I will try the seafoam also. As far as testing with a timing light I'm not sure how to test each one individually. Do you just put the lead on each one individually instead of number 1 cylinder? I will ask my buddy for some help with that one maybe he knows.
 

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Yes, just put timing light on each wire and point it at something, watch the flashes, if it is skipping you will see it, you aren't timing anything but the flashes :)

I have come across older distributors that developed a wobble which would cause misfires because of changing gap from rotor to cap contacts, but that's a long shot

Head gasket was mentioned because it was just rebuilt, every one has a Monday Morning now and then, lol.
 

FredrickPatton

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So im not sure what I did to fix the problem other than the timing was off slightly. Ive been checking everything over a couple times and some how its running better now. Im sure it was a combination of small things that got all panned out by me taking everything related apart and cleaning and replacing. But whatever it was, its running good now so thanks for your help!
 

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Good work :icon_thumby:

Thanks for the update
 

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