Rough Idle need help tried just about everything

Moe0813 · May 21, 2015

Hi guys,
Just picked up a 98 4wd ranger my 4th ranger. throwing code p0303
it has a rough idle but when driving theres no misfire or anything just at idle.
ive replaced the following
-Plugs
-Wires
-Coil pack
-mass air flow
-cleaned IAC
now heres where it gets confusing
i can pull the cyl 3 spark plug wire while trucks running and it will have no effect what so ever on the motor if i pull any other wire off the coil pack it hesitates and runs even rougher
same with the #3 injector i pull the connector and nothing happens if i pull cyl 3 injector it runs like crap.
So... i did the following i swapped inj 2 with 3
and the same thing if i pull the cyl 3(which was cyl 2's injector) wire no effect at all but if i pull cyl 2 (which was cyl 3's injector) it runs like crap so the injectors are fine..
im at a loss. if you have any ideas what i can do please let me know
forgot to mention im at 171k our taurus had 300k no issues same year same motor would really love to know where to start
thanks in advance
-Moe

RonD · May 21, 2015

Reads like low compression in #3, P0303 is a #3 misfire, but in computerese it doesn't mean #3 will be missing as in dead.
Computer has the CKP(crank position) sensor behind the crank pulley, there is a tone wheel(looks like a gear) on the crank, each tine a "tooth" goes by the sensor a pulse is sent to computer, this is where the RPM comes from and the spark timing and injector timing.
Because the computer can see the rotational degrees and velocity of the crank, it can tell if a cylinder is adding power to the crank or not adding power.
Not adding power would be a miss, but computer doesn't just set the P030x code right away, it has to be a repeatable miss and near the same RPM, load, MPH, and air flow.

Pulling spark plug wire or fuel injector for #3 and not having a dead miss means it wasn't firing anyway, so already missing.
Because your "miss" is at idle and seems to go away at higher RPMs then I would suspect compression, and it is a straight forward test, you know right away if #3 has a problem, or you can take it off the table as the problem.

You could have a small vacuum leak at the rear of lower intake, by #3, this will lean out #3 at idle but not at higher RPMs

Compression test needs to be done with all spark plugs removed, so starter motor can turn crank faster, and with good battery for same reason.
Cold or warm engine doesn't matter, in my opinion
Press gas pedal to the floor and hold it down when cranking engine, this allows air in but also turn off fuel injectors

Write down results
3.0l should be hitting 170psi per cylinder at sea level, 9.3:1 compression ratio

But point of compression test is not about high numbers, it is about averaging the numbers.
You take out the highest cylinder and the lowest, say you got a 145 and a 175, as low and high
Add up the other 4 and then divide by 4, say this came out as 168
Move over a decimal place, so 168 = 16.8, that's 10% of 168, make it 17 to be easy
168 + 17 = 185
168 - 17 = 151
So any cylinder outside that range is a problem.
The one at 145 would be an issue

Rough Idle need help tried just about everything

Moe0813

New Member

RonD

Official TRS AI

Sponsored Ad

Sponsored Ad

Staff online

Members online

Member & Vendor Upgrades

Latest posts

Truck of The Month

Recently Featured

Follow TRS On Instagram

Latest Album Photos

TRS Events

25th Anniversary Sponsors

Check Out The TRS Store

Sponsored Ad

Sponsored Ad

Sponsored Ad

Amazon Deals