coils testing good, can they still be bad?

[jgt]

Hi all, got a 99 2.5l ranger with 144k and original coils. replaced wires and plugs, cleaned maf, throttle body and fuel filter. Starts up great and runs perfect for the first 10 minutes, then idles rough and hesitates. I tested the coils and the primaries were .4 and secondaries were 13.4k. I cleaned up the ground contacts and swapped them front to back. Same results. I read this is classic symptoms of a bad coil and they can still be bad coils even though they test good. Should I replace? yah or nah? 1 or 2? thanks

 

[veefer800canuck]

Can't say exactly for Ranger coils, but sometimes, on motorcycle coils, when they get hot, they flake out, but test OK when cold.

So might be something for you to look at???

Maybe try swapping coils from a known well running truck and see?

Or get a set from the wrecking yard and try them.

 

[RonD]

All 4 primary's tested at 0.4ohms?

Seems a little low but within the 0.3-1.0 ohms spec

But your issue could be something else.
When first started the computer(PCM) will be in Open Loop(choke mode), it does not use the O2 sensors, and runs an air/fuel mix based on "tables" in memory, this will be a rich air/fuel mix, also a higher idle and advanced spark timing(choke mode).

The PCM uses the ECT(engine coolant temp) sensor to "see" engine temp.
ECT is a TWO wire sensor only used by the PCM, the engine also has a ONE wire SENDER, which is used for the dash board temp gauge, the two are not shared.

As the engine warms up(ECT info) the PCM will start to lower the idle and lean out the air/fuel mix, when engine temp reaches about 180degF the PCM will start using the O2 sensor data, the PCM is now in Closed Loop, it is called this because no outside data(tables) are being used.
O2 sensor shows rich, PCM reduces fuel, O2 shows lean PCM increases fuel.
This is also called a "Feedback" system.

From your description it sounds like there could be a problem when PCM switches to Closed Loop.
This could be something simple like a vacuum leak, in Open Loop the fuel mix is rich and idle is high so a small vacuum leak wouldn't be noticed, in Closed Loop it would be noticed.
Warm engine idle should be 750-800rpms, if higher you have a vacuum leak.
If you unplug the IAC Valve on warm engine at idle, the idle should drop to approx. 500rpms or engine may stall, either is fine, it means no vacuum leak, if RPM doesn't drop then you will need to find the vacuum leak.
PCV Valve hose is a very common leak point, check it carefully for cracks, these generally start failing at the 10 year mark.

With a 1999 you will have an OBD II system so can do a Live Scan to see what the O2 sensor is doing, it's numbers should be jumping around very fast, that is a properly operating O2 sensor, if it is a slow change of numbers replace O2 sensor it is worn out.
That is the Bank 1-Sensor 1 that jumps around, Bank 1-Sensor 2 is back behind cat converter so will change slowly, which is normal for that sensor location.
They are both the same sensor(part number) so can be swapped, the back sensor gets less carbon build up so tends to last the life of the vehicle.
The back sensors main purpose is to test if Cat converter is working to lower the emissions, so doesn't play a big part in air/fuel mix.

 

[hank857]

at 144K you could have some dirty/bad injectors! on my 94 it showed the classic symptoms of an ignition miss,idled good but miss under accell etc. Changed all ignition components still bad, took it to a shop to confirm work done all was good could not figure out. Said the injectors appeared to be good. Anyhow I just put new injectors in and the miss disappeared. had dirty bad injectors not ignition related at all.

 

[jgt]

yes all primaries at .4. We live in Hawaii, so there was rust on the bottom of the coil too. The saltwater is rough on the vehicles here.


thanks guys for the great info. yes, I leaning towards dirty injectors. Forgot to add important info.
-It started with an intermitent flashing CEL (p0302, missing #2 cylinder) RIGHT AFTER running the tank really low and filling up full tank at costco. Ran rough all the way home.
-popped hood, found a very badly cracked breather hose, changed it and flashing CEL gone, but solid CEL still on with same code
-The fuel filter was changed it was clogged up with dirt. (changed wires, filters, cleaned maf, throttle body)
-after that we cleared codes, ran good 10 min then rough again and po302 popped up
-changed plugs (cracked porcelain on plug, #2 driver side). The plugs themselves were clean, but had about 50k on them. again runs like a champ 5-10 min, then rough idle etc.
-BTW, didn't change the really hard spark plug on that side (#4 drivers) yet. but I read it's the 'waste' spark and not really necessary

So far only ran around the block, and no code yet. hesitates really bad at idle after warmed up. Rattled the PCV valve and it was good, but original. I will get fuel injector cleaner today and a new pcv valve since it's original. Lucas is on sale at napa. We have ethenol gas here in HI. Was hesitant to use seafoam or anything really strong. I heard you may have to change the plugs after using it, and honestly, I never want to change those plugs again. thanks for all the info...i will keep an update.

 

[martin]

As you appear to be missing on number two, try exchanging injectors one and two and see if the code moves with the injector.

 

[OilPatch197]

at 144K you could have some dirty/bad injectors! on my 94 it showed the classic symptoms of an ignition miss,idled good but miss under accell etc. Changed all ignition components still bad, took it to a shop to confirm work done all was good could not figure out. Said the injectors appeared to be good. Anyhow I just put new injectors in and the miss disappeared. had dirty bad injectors not ignition related at all.

Try a couple of tanks of gas treated with Techron Concentrate Plus. Techron Concentrate Plus is 10X levels of Techron found in Chevron gas + upper cylinder lube and other proprietary ingredients(PEA).

 

[CheapRanger]

Just replaced both my originals (one was even cracked) and both tested good cold, but would misfire when hot.

 

[jgt]

been running lucas fuel injection cleaner for a day now. still rough, took it out to highway. Flashing CEL, now P0302 AND P0136, which is new. Can the bad fuel injector cause this O2 sensor code to come up? What do you think, go ahead and pull the #2 injector? or look at 02 sensor? thanks

 

[Andy D]

too early for the cleaner to work. There is a tool called a noid light to check if the injector is working.You could probably use a meter some how too. Also have you eliminated a low cylinder? Swapping coils and /or injectors to another cylinder to see if the trouble follows the part. Most trouble shooting is testing parts and pieces to verify their condition or eliminate them as the trouble. Throwing parts at the trouble in hopes that you fix it is just plain bad. Also, age is a factor, EFI is fairly bomb proof, but connections get dirty and frayed. Salt is a problem here too. Wet wires turn to green slime after 15 yrs of damp sea air. Its an old car thing ,grounds especially.

 

[adsm08]

Ignore the O2 code. Your misfire is putting lots of oxygen down the exhaust, confusing the sensor. The computer interprets this as a fault with the sensor.


Yes, a coil can ohm test ok and be junk. Resistance test is the bottom of the barrel for electrical tests. It's the equivalent of throwing a dart at a phone book to pick your future spouse. It tells you something is there connecting the two ends of the line, it doesn't tell you how good it is. A single strand still hanging on will not operate, but will pass resistance.

To add to the confusion one tower of a coil can make a cylinder miss while the other one on the same coil still be working properly.

Take your plug wire for the affected cylinder, swap it with the wire on the paired tower of the coil, and see if the miss follows.

 

[jgt]

Ignore the O2 code. Your misfire is putting lots of oxygen down the exhaust, confusing the sensor. The computer interprets this as a fault with the sensor.


Yes, a coil can ohm test ok and be junk. Resistance test is the bottom of the barrel for electrical tests. It's the equivalent of throwing a dart at a phone book to pick your future spouse. It tells you something is there connecting the two ends of the line, it doesn't tell you how good it is. A single strand still hanging on will not operate, but will pass resistance.

To add to the confusion one tower of a coil can make a cylinder miss while the other one on the same coil still be working properly.

Take your plug wire for the affected cylinder, swap it with the wire on the paired tower of the coil, and see if the miss follows.

thanks adsm08, can you clarify? I'm a newbie, so please forgive me. I will have a friend help me on Wednesday. Which side on a 2.5l is the waste side? I read it's the driver's side? So I take the spark plug wire on the rear coil (going to passenger side) #2 and swap it with #3 on the coil, is that the paired coil? And just start it up and drive it? How about the other #2 that goes to the passenger side? Just leave it? I'm not coming up with a code unless I drive it 15 minutes or so.

Also my haynes book ohm resistance is different than the autozone one here. If I follow the autozone, they say secondary should be 6,500-11,500 ohms. Haynes said 13.4k is fine. Anyone got the proper spec? thanks, will report back with solution, I'm sure it's a simiple one. Taking the intake manifold off to get to last plug and maybe check injectors too, anything I replace do while that's off? thanks.

 

[jgt]

As you appear to be missing on number two, try exchanging injectors one and two and see if the code moves with the injector.

I tried to swap 1 and 2 without taking the intake manifold off. can't get the to harness on #2. will have a friend take it off to do last plug and get a clear shot at the plug holes and maybe check injectors if coils test ok. I nearly stripped #3 on driver's side, crossing my fingers that I didn't, but from being careless I suppose.

 

[jgt]

my friend pulled the spark plug wire from the #2 spark plug and layed it to ground, while the other end of the wire stayed connected to the coil. He had me start the engine and it sparked up. We repeated it on the other coil and #2 and it sparked up. He said this shows that there is spark going from the coil. Is this a good test?

update: after replacing all spark plugs and double checking gaps, he found a disconnected vac hose under the intake manifold. he connected it with new clamps. He looked at the injector o-rings and said they looked good. I forgot to tell him to swap #1 and #2, he was pretty sure it was the hose, and buttoned everything up. Still flashing CEL code p0302, and now p0171. running real rough.

 

[jgt]

Bad compression test on 2.mechanic friend said bad valve. Bummed.lesson learned for this newbie