Have an '06 Sport,3.0. Keep getting multiple misfire, misfire during first 1,000 rpm,cylinder 1&2. When the ac is turned on the misfire disappears. Pickup is parked because it's dangerous to drive. Any advice is appreciated.

 

Welcome to the forum

First test should be a compression test
Cold engine, remove all spark plugs first
Test compression in each cylinder, write it down

Expected is 160-170psi, but all gauges are different, so what you want to get is an average psi number even if its 140psi average

There is a TSB for 2004-2006 3.0l Rangers, seems there were some bad valve seats installed at the factory and they could shift positions which caused a loss of compression and burnt exhaust valves over time
This is why you need to do a compression test FIRST, to either Confirm its the problem, or Confirm its not the problem and move on to a spark or fuel issue, otherwise you are just wasting time and money

If you get 4 cylinders with 150-155psi and 2 with 135psi or less then you have leaking exhaust valves in those 2 cylinders, and will need to change/repair the heads

TSB seen here: https://www.therangerstation.com/for...n-tsb-05-26-3/

No, there is no recall or warranty on this

 

Compression tests have been done,no loss of compression. This is also the second engine this little truck has had,the first burned a valve and the shop I used (no garage here) put on heads that had a valve spring break and plant the valve head squarely through the piston head. That clown told me 'oh well' pretty much and will face no consequences because of the laws here in Texas. This new engine had 60k verified but still the same problems. This has me wondering about the harness?

 

What year was the replacement engine, or was it new/rebuilt?

Well you can rent a NOID light kit
NOID light plugs into fuel injector and its 2 wire connector
Start engine
NOID light will flash each time injector gets the pulse from computer to open, intermittent pulse would indicate wiring issue or computer issue

Injectors all get 12volts with key on, they share the one 12v wire, computer grounds each injector to open it, each injector has its own ground wire back to computer


If you have or can borrow an old time Timing Light you can test each spark plug wire to see if spark is steady, no missing pulses

If you believe compression is not the issue, I would still test it, then all that is left is fuel and spark to cause misfires
Computers make engines more reliable and simpler to work on, but they don't change the way gasoline engines work
The Big three, well only three really, lol
Compression
Spark
Fuel
If one(or more) of those is missing then you get a misfire, as simple and as complicated as that


Liquid gasoline can not be ignited by a spark, yes the movie guys take liberties with that fact, lol
Only gasoline vapor can be ignited by a spark
Compression vaporizes the liquid gasoline that comes out of the injectors(or carb Jets) so the spark plug can ignite it
That's why compression is important
Just FYI

 

Yep,and I ordered a new ecm,had it and key programmed. Why does the misfire disappear when the ac kicks on? Could there be an O2 sensor screwing up?

 

No on the O2 sensor, O2 looks at the exhaust from 3 cylinders, so its not precise enough to cause specific misfires

You just don't feel the misfires with AC on, which is odd but not definitive, AC on causes computer to raise RPM by 50-75 using IAC Valve, so not really anything related to misfires Big 3
Fuel
Spark
Compression

I would have done compression test before spending any more money

 

Already have,all are 140-160 with no blow by. New coil pack,New Bosh Platinum and New wires,that's why this is so mysterious.

 

All piston engines have blow-by, that's normal operation, and why there is a PCV system

140-160 is not a great range, more than 10% difference
Were 4 at 160 and 2 at 140?
That would be more helpful

 

Two at 140 Ron but not 1&2. I put a compression tester on both cylinders today and blow by was for the most part negligible. PFM as we said in the Navy.

 

The misfire codes are not always accurate, jut FYI

Firing order is 1 4 2 5 3 6 so 6 4 or 5 could be the actual cylinder misfiring
Which is why computer has P0300 or P0316 codes to use
And because its Waste Spark, disconnecting a spark plug wire kills 2 cylinders not just the one, so can't test it like with a distributor engine

I would put a teaspoon of oil in the 2 lower PSI cylinders and retest, number will always go up, but if its not higher than 165psi then exhaust valve is leaking, could be intake but exhaust 99.9% of the time

It may not be a compression issue at all, but it really is a waste of time and money to skip over lower compression cylinders as "acceptable" and move on

 

Okay Ron,I'll look into it this weekend. Yep,the oil will tell if there's bad rings.