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Struggling with Rough Idle, can't think of what to do next


ejon101

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Hey Guys, I've gotta 1999 Ford Ranger 4.0 5spd, 195k mi. When I bought the Truck, it had a really rough idle and occasional misfire under load. I've tried countless things, but still can't manage to figure out wtf is wrong with the thing.
(current) Symptoms are: Misfire under load, extremely rough idle, occasional oil pressure drops (I'm running 5w30 and the gauge just dips sometimes. It's prob just the sender but I'm mentioning it anyways), P0302 & P0305 codes.

What I have done: Head gaskets, Intake Manifold Gaskets, Upper Intake Manifold Gaskets, Exhaust Gaskets, Cat-Back Exhaust, Fuel Filter, Oil&Filter, Air Filter, Spark Plugs, Spark Plug Wires, Distributor, Seafoam (through the throttle, Oil, and Gas), Injector Cleaner (through the gas). I was told to try injectors so I pulled everything out and all injectors are getting proper signal from ECU, and all injectors have uniform resistance.

Before I preformed the Intake Manifold Gasket I had a nasty vacuum leak coming from a snapped intake manifold bolt. That vacuum leak caused Banks 1&2 lean codes, but they haven't returned in the 150mi I have driven since the job.
 


DannyG

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Have you tested the flow of the injectors? The cleaner may not have corrected enough. They could be clogged and not misting the fuel as they should even though all the digital/electrical sections are within specs.

Also, have you inspected the computers physically? Looking at the board for hot spots (burnt/dark coloring), etc.

Just some suggestions as it is mostly computer run.
 

91stranger

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If your oil pressure gauge Is dipping then I would replace the $10 sender and see if that solves it. If not, then you may have another issue going on. I'm not familiar with the 4.0 liters anymore. I know I just swapped a camshaft synchro on my 3.0 b/c of the oil pressure dropping. I know those 4.0's were notorious for timing issues but I don't remember what years were worse than others. I'm sure RonD might have some good input on this.
 

ejon101

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Have you tested the flow of the injectors? The cleaner may not have corrected enough. They could be clogged and not misting the fuel as they should even though all the digital/electrical sections are within specs.

Also, have you inspected the computers physically? Looking at the board for hot spots (burnt/dark coloring), etc.

Just some suggestions as it is mostly computer run.
I will inspect the ECU, good idea. How would you test the flow of the injectors?
 

ejon101

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If your oil pressure gauge Is dipping then I would replace the $10 sender and see if that solves it. If not, then you may have another issue going on. I'm not familiar with the 4.0 liters anymore. I know I just swapped a camshaft synchro on my 3.0 b/c of the oil pressure dropping. I know those 4.0's were notorious for timing issues but I don't remember what years were worse than others. I'm sure RonD might have some good input on this.
I was thinking of just replacing the sender, then connecting the sender to an aftermarket guage that would actually give me an exact oil pressure reading, rather than the dummy guage on the cluster
 
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RonD

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If there are misfire codes then computer will switch to Batch Fire injection, opens 3 injectors at the same time instead of sequential injection, so that can take injectors off the table.

Miss at idle only can be dirty injector tips, can of seafoam in the tank cures that
General misfires when driving(you only notice it under load) can be a few things, coil pack, but that would usually be just matched cylinder misfires, i.e. 1/5, 2/6, 3/4 as each pair has its own coil, but it is possible for 2 coils to fail, look for cracked case on coil pack


Even though its a pain I would pull all spark plugs and do a compression test, take that off the table FIRST so you don't waste time and money on non-fixes if there is a compression issue.
4.0l OHV should run 155psi average compression pressure, cold engine, all spark plugs remove for good cranking speed
 

ejon101

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If there are misfire codes then computer will switch to Batch Fire injection, opens 3 injectors at the same time instead of sequential injection, so that can take injectors off the table.

Miss at idle only can be dirty injector tips, can of seafoam in the tank cures that
General misfires when driving(you only notice it under load) can be a few things, coil pack, but that would usually be just matched cylinder misfires, i.e. 1/5, 2/6, 3/4 as each pair has its own coil, but it is possible for 2 coils to fail, look for cracked case on coil pack


Even though its a pain I would pull all spark plugs and do a compression test, take that off the table FIRST so you don't waste time and money on non-fixes if there is a compression issue.
4.0l OHV should run 155psi average compression pressure, cold engine, all spark plugs remove for good cranking speed
@RonD When I tore down the engine for the intake manifold repair, I replaced the head gaskets. I've been meaning to preform a compression test to confirm the head gaskets sealed properly. Coil pack was already replaced, Seafoam has already been run. I might be able to do the compression test tonight, I'll keep you posted.
 
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ejon101

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When I tore down the engine for the intake manifold repair, I replaced the head gaskets. I've been meaning to preform a compression test to confirm the head gaskets sealed properly. Coil pack was already replaced, Seafoam has already been run. I might be able to do the compression test tonight, I'll keep you posted.
@RonD Assuming that the compression test passes spec and there are no other vacuum leaks, could a loose/skipped timing chain be to blame?
 

RonD

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No, and not likely on the 4.0l OHV, and compression test would show that in a big way, you would see under 130psi overall compression if cam/crank timing is not correct

If you have a lower compression on #2 and #5 cylinders, do WET test on all cylinders, add oil via spark plug hole and retest all cylinders
 

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