1994 4.0 OHV Running Poorly

[DasBaldDog]

Okay. Short and sweet

4.0 OHV
145,000 miles

Engine idles at 550 or so rpms
When you goose it at idle, it hesitates (like the engine just stops for 1/4 a second)

General running and you get low power, poor fuel economy (8 mpg or so)

I know the cat is in rough shape and already have plans to swap it out.

Fuel pressure is 38 psi engine off, and 30 engine on (I think this was it, I can't remember and plan on doing it again tonight)

Anyways

Symptoms

Low Vacuum (14" Hg)
Low Power
Horrible acceleration in 3rd, 4th or 5th gear.
Poor Gas mileage
Slightly rough idle
1/4 second hesitation on the throttle
Oil around the threads on the #5 spark plug (with a fuzzy white head) and another plug (can't remember which one) was oil fouled.


Done So Far

Swapped Fuel Filter
Cleaned MAF
Replaced Spark Plugs
Replaced Spark Plug Wires
Cleaned Throttle Body
Checked operation of MAF
Checked operation of TPS
Retorqued Lower Intake Bolts (using the apparently incorrect outside - in method)
Retorqued Upper Intake Bolts
Checked for blown head gasket (Chemical test came back negative)
Tested Fuel Pressure (38 psi before start up and 30 psi at idle)
Checked compression (140 - 150 psi on the drivers side bank... couldn't get the damned compression tube into any of the cylinders on the passenger's side)
Sprayed Brakleen all over the motor and no change in engine.

Some other stuff I can't remember.



Truck has NO EGR system. I checked, thoroughly.


What I think could be the problem.
-Low fuel pressure could be either dying fuel pressure regulator or a leak between the fuel rail and intake manifold, maybe a bad gasket there.... I have no idea how to check to see which it is.
-Low Vacuum leak could be a leak between Manifold and Motor (but I retorqued those bolts and there was not a single sign of improvement) or it could be caused by excessive backpressure from the exhaust
-Oil fouled plug could be just a worn valve stem seal or could oil be leaking from the intake manifold?

I'm not afraid of spending the cash to buy replacement parts, I just don't want to replace what isn't broken or breaking.



Anyone have any ideas what I can do next? Really need this truck to be working before the even worse weather hits.

 

[rickybobby]

i would start with replacing your spark plugs. everything else you describe sounds acceptable. if you think your cats are plugged, the best way to find out is to cut a small slit in front of each one, one by one front to back, driving it after each cut to check for improvement. when you are done with that take it to an exuast shop where they can weld the slits closed and replace any bad cats you may find

 

[DasBaldDog]

Spark plugs are brand new.

I have a new direct fit cat in my garage that is going in this weekend.


The spark plugs would not have an effect on the oil fouling or low vacuum readings.

The cat would in theory MIGHT fix the low vacuum by reducing engine backpressure, but would still leave the oil fouling problem... which may just be a valve stem seal issue.

 

[DasBaldDog]

I found that the hose that goes from the Evap Canister to the fuel tank was rust rotted at a connection at the back bottom of the engine bay.....

Does anyone know if this, or if the purge solenoid were not opening would cause any of the issues I am seeing?

The hose between the throttle body and Purge solenoid, and between the solenoid and cannister appear to be in good shape.



Oh and fuel pressure at idle is 39 psi.

 

[Wakeboy1337]

Your pcm is most likely bad. Code Scan it and see if you get the code "998"

If you do, its time for a new computer. 15 bucks on ebay.

I thought all 94s had EGR? Hmm maybe I was wrong.

 

[FIND]

have you verified operation of all injectors? Also, I am with the other guy. Have an exhaust shop check your cat

 

[DasBaldDog]

Wakeboy,

Please explain why you think it is the PCM.

As far as I can tell, other than possible timing issues, not much points towards the PCM.


Find,
Please read above, I verified the cat is failing and have a replacement in my garage ready to be swapped in this weekend, but as verified by the vacuum gauge, it is not causing sufficient enough of a problem I do not believe.

Faulty injectors would also not cause low vacuum, or oil fouled plugs.

Thank you the help though.

 

[FIND]

Find,
Please read above, I verified the cat is failing and have a replacement in my garage ready to be swapped in this weekend, but as verified by the vacuum gauge, it is not causing sufficient enough of a problem I do not believe.

Faulty injectors would also not cause low vacuum, or oil fouled plugs.

Thank you the help though.

yes, they would. An injector spraying too much fuel can leave you with what appears to be oil fouled plugs. A plugged cat will cause excessive backpressure which means that your cylinders will not completely evacuate exhaust gasses which will lead to lower vacuum. The plugged cat can even cause the plugs to appear oil fouled, as exhaust gasses will stay in the combustion chamber and can coat the plug, or you can have incomplete detonation which will leave the same kind of film. Faulty injectors pouring fuel can also cause incomplete combustion because of a bad mixture. Also, sorry I missed that you already verified you had a bad cat.

I have had vehicles in my shop that have displayed your exact symptoms, ending up being simple fixes like a cat or injectors.

 

[DasBaldDog]

Well as the cat is to be replaced anyways, how am I to check the injectors?

Won't get to it today, today is Clutch and Slave day, but might get to the injectors soon.

 

[MrShorty]

I think if this were me, I'd replace what you know to be bad (the cat) and see what happens after that. There's probably more to the problem, but I like to eliminate known bad parts so that they don't muddy up the diagnosis of other parts.

 

[FIND]

simple way to check injectors, big long screwdriver against the injector, ear to the screwdriver, listen for the click.

 

[donniekl]

I have a '97 4.0 OHV with a lot of your problems as well. Virtually no power at highway speeds unless the engine is wound up above 3K.

I know you said your cat was bad, but how did you determine that? I'm looking forward to seeing if a new cat helps your issues.

 

[koshie6]

DasBaldDog, I have the same motor and exact same symptoms and problems right down to the 550rpm idle. I have changed, MAF, fuel pressure regulator, TPS, Idle control, plugs, plug wires, coil pack, fuel filter, O2 sensor after cat, purge solenoid and valve (oh and the coolant temp sensor). and still get about 6-8 mpg. I spend over $200 on gas a week and so want this issue resolved. If you can let me know if you have repaired your problem and what the resolution was, it would be greatly appreciated.
Thanks

 

[donniekl]

koshie, my problem ended up being a bad O2 sensor, but I also replaced a lot of the parts you described. Sounds like you might want to look for a vacuum leak. What codes are you getting?

 

[koshie6]

Now i'm only getting a purge valve, purge solenoid, and o2 sensor code (both have been changed) . The o2 sensor in question is brand new but is getting 12 volts to it. To my understanding its only supposed to range from .2mv to .9mv, and there is one that registers no power at all. Glad to hear that you got yours fixed. The fuel consumption on mine could be no more than 6-8 mpg and I cant wait to get it fixed!

 

[MrShorty]

The o2 sensor in question is brand new but is getting 12 volts to it.

The O2 sensors on these are heated, meaning they have a small electrical heater in them to get the sensor up to temperature faster. The O2 sensor should have a constant 12 V to it to run the heater. The O2 sensor's output is supposed to range from 0.2 V (lean) to .9 V (rich) when the mixture is being controlled correctly. You'll need to look at a wiring diagram to see which wire is the o2 sensor signal wire. If you have a constant +12 V on the signal wire, that suggests a short between the o2 heater power and the signal wire.

 

[koshie6]

Mr. Shorty, i just joined the site and i'm loving the input so far! Was messing around with it yesterday and aggressively pressing the throttle produces black smoke out the pipe.

Here's what i found out about the truck, it originally had a 3.0 automatic, and had a 4.0 manual put in. The odo is not working.

Standing in front of the truck facing it, the o2 sensor closest to the front on the driver side only has 2 wires going to it whereas the other 2 (the one on the front left and the the one post cat) have 4.

My next move was to change the wiring harness and computer. I figure a new harness should have all 3, 02 sensors having 4 wires, the new computer would be to rule out anything thats going on with the old one.

Thanks again in advance.

 

[donniekl]

Is it possible that you have the wrong type of O2 sensor instead of a bad one? I looked at my wiring diagram and all O2 sensors should have four wires. What does the harness look like that the O2 sensor plugs into? If it has four wires then I think you would just need the correct sensor. Take the four-wire sensor from behind the cat and swap it with the two-wire sensor and see if it gets better.

The black smoke and poor mileage indicates your engine is running VERY rich. I drove with a bad O2 sensor for a little bit and it ended up breaking my spark plugs so I hope you get it fixed soon!

 

[koshie6]

Thats why i want to replace the harness. The other 2, post cat and passenger side, have 4 wires from the harness. The driver side only has 2, red/yellow and green/orange. That may just be the whole reason for my 6 miles a gallon.

Yesterday I disconnected the o2 sensor after the cat and used the plug that powers it (with 4 pins) to power up the driver side o2 sensor. That produced even more black smoke.

As soon as i can get my hands on the proper harness i will let you know the outcome, unless you have any other suggestions.
Thanks