JimmyE

So I have a 1998 ZJ with the 4.0. It was running fine until recently. When starting it you have to crank it for about 10 seconds before it will try and start. Now once started it idles really good. But once it goes into gear and start to drive it chugs really bad. I also notice that the rpm gauge will jump sometimes when it chugs. It is not throwing any codes and I just gave it plug, wires, cap, and rotor thinking that was the problem. I know it could be a lot of different sensors but before I start throwing parts at it I would ask here for any other ideas.

dave1123

Welcome to CF!

When you say it chugs, do you mean it feels like it's bogging down, like running rich? Or is it like getting the gas? Check the vacuum hose to the MAP sensor and your actual fuel pressure. You can rent a gauge from Autozone or Advance Auto or O'Reilly's. You should be holding around 50 psi, especially after you shut it off.

JimmyE

I would say the chugging feels more like a miss. I did notice when I went to change the injectors I pressed the release valve on the fuel rail and not a lot of fuel came out and it didn't seem like it was under a lot of pressure.

SatiricalHen

Did you disconnect the battery and reset the computer after doing all of that? Get a gauge and check the pressure on the rail, you can rent them at parts stores.

JimmyE

Yeah I reset the computer. Im going to go get the fuel pressure tester.

JimmyE

I also notice that if I put it in drive and hold the brake and give it gas it chugs really bad but if I do it in reverse it doesn't.

dave1123

Wait....you didn't say anything about changing the injectors. Were they new or used?

JimmyE

I went to change to the neon injectors but ended up just changing O-rings. Guess I should have worded that better.

JimmyE

So I tested the fuel pressure today and it sits at about 48psi even when its chugging. Turn it off and it drops to 11psi almost instantly. I still cant figure out why it does its chugging in drive but not reverse. Anything else I can try?

dave1123

NO!! It should not drop pressure so quickly. I'm thinking you have a sticking or leaking injector, OR maybe you have a bad electrical connector at an injector leaking voltage to ground instead of to the PCM. The injectors are supplied with 12 volts when the key is on, and the PCM cycles them by grounding their circuits. Check for frayed wires at the connectors. It's possible this grounding is intermittent and only happens when the engine torques. At idle or reverse, the engine torques the other way.

Whatever the cause, the fuel pressure should hold at or near 50 psi for several hours after shutdown. I've heard it should hold at least 25 psi overnight.

JimmyE

Ok thanks for all the help dave1123. Ill start trouble shooting the injectors and the wiring hopefully I find something before I set it on fire. LOL

JimmyE

Well a little update. I changed out the fuel pump and it starts right up now. But it still sputters when driving it. Anymore suggestions I can try?

97grand4.0

The classic drop off when you shut the key off is the hallmark of the pressure regulator/filter. It should hold pressure for a good long time. Is it still doing this? Mine did the same thing, just plummet when key off. If I recall, it should hold at least 25 lbs after something like 10 ins but I know that's not quite right. If so, I would think you need to pull the pump again and do the filter regulator if you haven't already. Dont feel bad, I did the job 2x myself to fix mine.
That said, the crank sensor and the CAM sensor (distributor switchplate) are the next items up for bid. Mopar only on these!!! CAM sensor can be had at a junkyard, when they go its long cranking usually, but not rough running.
I would mist it while running with a water bottle and watch for fireworks, this works best at night.

JimmyE

My pressure is good now. It doesn't drop fast after turning it off. I'm going to start replacing the crank sensor just to see if that helps the driving issue. Thanks for the tip.

JimmyE

Well its fixed. I went to pull the CPS and the old one wasn't even bolted in.