you do use a SCANNER to check codes - key trick is lacking...

talk to your mechanic - with his scanner looking at live data he may see a red flag something out of spec.

look at codes duh but also ECT, Fuel trims, o2 voltages, MAP and TPS and their relationship..


A SWAG - check fuel pressure THEN leaving KOEO- observe fuel pressure drop - ie fuel drops Xpsi in 5min- post that- sounds like starting stumble missfire issue...

 

First, you might want to find a better mechanic. An unexplained 30-40% drop in gas mpg's is most often caused by an O2 sensor fault, either a failed sensor, a blown O2 sensor fuse, or wiring issues. The O2 sensors can be tested and their functions obsserved with an OBD-II live data scanner or scanner App. Test the Coolant Temperature Sensor also. If you buy new O2 sensors, only buy NTK brand, do not buy Bosch. Spark plug wires can be tested, you might want to test the ignition coil also.

The start and hesitation issues is most likely a failed check valve in the fuel tank. When a 1997 to 2001 Cherokee won't start on the first try day after day, the first thing to suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.

POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.
2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

There are a few solutions to the problem -

1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.
2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.
3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.

If you do replace the fuel pump assembly, purchase a high quality OEM style pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, and Precision don't hold up and will fail completely within 12-18 months.

 

3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

Confused on this point. In reading elsewhere, I'd seen the check valve described as part of the fuel pressure regulator, which is an individually replaceable part of the fuel pump assembly. Is this correct, or no?

 

Misfiring and hesitation on startup can point to a sticking injector, dripping fuel into the cylinder as it sits. Injector problems could also account for a fuel mileage drop.

 

Thanks for the replies!

I was aware of the check valve situation, as I diagnosed that problem about seven years ago (severe drop in fuel pressure at the rail after engine was turned off, and the drop in pressure doesn't happen when the line back to the tank is pinched off, so leaking injectors are not the cause). I knew that causes the delay in starting, which I deal with by turning on the ignition for about 3 seconds to allow fuel pressure to build up before cranking - kind of like starting a diesel and waiting for the glow plugs to do their thing. But I didn't think about the fact that the check valve situation could also explain the slight misfiring and stuttering for the first 10 seconds or so of running - makes sense - thanks for pointing that out.

As to the poor economy situation: I used my OBD-II App and looked at fuel trims and O2 voltage last night and all checked out fine. I tested at idle, 1500 RPM and 2500 RPM. Both long term and short term fuel trims were right around 1.5% except when changing RPM, and even then the short term trim just fluctuated for a moment to around 3 to 4% and then dropped back down. And the O2 sensor voltage varied between 0.1V, 0.4V and 0.8 volts as it should. I couldn't check the speed of the oscillations as I don't have a computer based scanner with graphing capability. So it's possible that my O2 sensor has become sluggish and I know this can lead to poor mileage.

Given the fact that the plug wires, distributor cap and rotor all are at least seven years old and have a solid 60,000 miles on them, I will replace them and see if that helps. Then if that doesn't get me back to around 15-16 mpg I'll probably go ahead and replace the upstream O2 sensor (with either a Mopar one or the identical NTK one - I went through the Bosch O2 sensor nightmare back in 2011 and wrote about my experiences (a new Bosch sensor from Autozone every few weeks until I caught on to the problem with the wrong resistance level in the Bosch's heater unit causing them to prematurely fail) on the Jeep Cherokee forum back then and also on this forum with my last post about the Bosch O2 sensor situation appearing here: https://www.jeepforum.com/forum/f13/why-mopar-over-bosch-o2-sensor-1049729/index5.html#post13834581

Cheers,

John

P.S. Any preferences or recommendations as to the best wires, cap and rotor to buy?

 

Old tune-up parts can definitely cause a drop in mpg's.

IDK if there is a "best" brand for tune up parts, but usually the stores have really cheap stuff, cheap stuff, and good stuff. Cheap parts are cheap for a reason, by the good stuff. You can never go wrong with brand name tune-up parts like Denso, NGK, or ACDelco.

Buy basic Champion or NGK spark plugs. Fancy spark plug coatings or Spitfire type gimmick plugs are a waste of money for Jeep 4.0 and can cause drivability issues.

 

A quality cap and rotor will have Brass construction.
NAPA will have quality parts,but you have to ask for them,as there are different teirs to their parts.

 

Their Echlin ignition parts seem to be their high end ones with a three year warranty for the cap and rotor. Their "Mileage Plus" line only carries a one year warranty. Might buy the Accel cap and rotor that Autozone offers - definitely a well known name brand and a few bucks less as well.

 

Hellow, I just bought a 96 cherokee classic with a 4.0. I had a leaky injector with fuel in the oil and oil light on. No gauge it just has a light. Fixed injectors, oil and filter change and new sending unit and oil lights still on. Ran the feed wire from the light to ground and it comes off and goes off when ungrounded. I ran a oil pressure test in place of the sending unit. At idle it dropped to 12psi but it took it 30 mins of idle to do it, so now im a littel confused. And advice?

 

13 psi is the minimum oil pressure specification in the Factory Service Manual. Lots of Jeep have run lots of miles with low oil pressure. You could try a different oil pressure sensor and see if it reads slightly higher. Depending on where you live, you might want to run a heavier weight oil.

 

MSD 8228 ignition coil
NTK o2 sensor upstream Wo133-1808727
NTK o2 sensor downstream Wo 133-1843736

 

Thanks a bunch, yeah I figured it was not dangerously low and the sending unit is just being sensative. I will try a thicker oil

 

MO30 Distributor cap
MO31 rotor
Both from NAPA

 

Ok so I drained the 10w30, stuck 15w40 and at hot idel it never fell below 14psi yet oil light is still on. I called orielys and autozone and they both say thats the right sending unit and its my engine. The sending unit shows continuity when its off the vehicle and no pressure. Taking it to a mechanic tomorrow...let him try to figure it out