1992 Ranger 3.0 idle problem/ intermintant CEL

frank1530 · May 9, 2015

Hey all ,
Its been a while since ive been on here i sold my 86 and bought a 92. History on the truck its a bare bones 3.0 auto custom reg cab. No ac, manual everything. It was a government surplus truck and has documented 98k miles.
Ive done a ton of work to it and I just cant figure it out. It idles on the low side(dont know exact no tach) and seems to surge sometimes. Never stalling out. It also shows the check engine light once in a while but turns off and doesnt store codes. Since Ive had truck i have replaced:

Plugs
Wires
Distributor cap
rotor
IAC
TPS
MAF
CTS
water pump
steering pump
radiator
all hoses
thermostat
and full exhaust
All parts have greatly improved the running and most were probably needed
This truck sat for a while

truck seems to run good at speed but will idle rough mainly in park a little less in drive. The only other note is that the trans shifts hard from 1-2 all other shifts are good. I also checked compression and all cylinders are 155-160.
Any help will be greatly appreciated. This truck is quickly turning from a great deal to frustrating.

RonD · May 9, 2015

What do the spark plugs look like?
Nice light brown/white
Or darker
If an engine sits for long periods the valve guide seals dry out and crack, so you can get a bit more oil in some of the cylinders, this can cause misfires at lower RPMs
Wider gap on plugs help idle and lower RPM performance, 3.0l shows 0.044 inch try 0.048

Can of Seafoam or similar cleaner in gas tank can help idle, dirty fuel injector tips tend to drip fuel at idle instead of spraying.

If you have a vacuum gauge see what that tells you about engine condition, if you don't have one then I would spend the $20 for one, they are one of the better diagnostic tools for any engine.
Good read here on the tests: http://www.gregsengine.com/using-a-vacuum-gauge.html

Does it miss less on startup, cold?

frank1530 · May 10, 2015

Thanks for the response
The plugs i took out all looked good and light brown.
The new motorcraft ones i put in don't have much residue yet only couple hundred miles on them.
The idle is better cold, not perfect. It gets worse as temp goes up.
Most noticeably in Park. If I put it in Drive it smooths out a bit.

frank1530 · May 14, 2015

UPDATE: both good and bad!

So im still having the rough idle, and now im getting either shuddering or missing at higher rpm under load. seems like may be a fuel issue, as i still havent replaced filter or bench tested injectors. i checked the electronics of injectors and the were all firing. As RonD suggested i checked vacuum and got good numbers. Now the interesting part is i live in CA and it was do for smog. I took it in expecting failure but kinda wanted baseline. Much to my suprise it passed and by a pretty good margin as follows.
CO2 O2 HC CO NO
15MPH 14.8 0 98 of 127 .16 of .55 443 of 721
25MPH 14.8 0 42 of 75 .08 of .69 94 of 623

Im hoping someone can look at these numbers and help kind me into finding culprit. One side not is the original factory exhaust was rusted out so I replaced it with Walker components i installed a new intermediate pipe stock muffler then before axle down pipe. Is backpressure problem possible with this set up.
Thanks, frank

RonD · May 14, 2015

No 4-stroke engine was ever designed to have back pressure, 2-stroke yes, lol.

Factory exhaust is tuned to generate highest negative pressure at mid-RPM range, after market headers are usually tuned for lower RPM negative pressure peak.
Tuned exhaust causes a siphoning action called scavenging by creating a high velocity in the pipe nearest the exhaust valve and then dumping that velocity into a larger pipe(collector), when the high velocity hits the larger pipe it creates a pressure drop, in the case of V6 the pressure at the other 2 exhaust valves drops so when they open exhaust is pulled out reducing the amount of power needed by the crank to push it out.

The diameter and length of the manifold pipes and the size of the collector pipe are what decide when the lowest pressure will occur in the RPM range, there would be a specific RPM with lowest pressure and RPMs nearest that RPM would be the range.

When people just put on larger pipes, Free Flowing, and found they had lost power, they wrongly assumed that engine needed "back pressure", what they had done with the larger pipes was to remove the "tune" and lower pressure it created, so lost that added power from scavenging.

Not sure how your system was changed, or if unbalanced exhaust pressure would create you issues.

1992 Ranger 3.0 idle problem/ intermintant CEL

frank1530

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RonD

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frank1530

New Member

frank1530

New Member

RonD

Official TRS AI

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