jeepguy93

Brother in law just bought a jeep 89 renix xj 4.0 auto.

While driving it will randomly hiccup and drop to 0 rpms. But not for ling its a tenth of a second. It'll drive normally no issue.

Also at idle it will idle really low 200-500 rpm and stutter and just shut off at stop lights etc, or when you press the gas it'll stutter to start or just shut off.

List of new parts

Fuel pump
Fuel filter
Cps
Tps
Cleaned iac
Cleaned throttle body


What could be causing this?!?

Radi

I assume it was doing this previously and didnt' change when the parts listed were installed?
A dropout that lasts just a second or so is often ignition failure. How do the plugs/wires/cap/rotor and coil connectors look? Also grounds and the big C101 connector may be worth refreshing.

jeepguy93

New ignition coil as well. Planned a tune up today

67 GMC

Mine did the "drop to 0 RPM while driving a few times" which was scary. It turned out to be the CPS wire. I caught it just starting to melt on the exhaust manifold. I guess I didn't route it properly so I zip tied it so it can't do that. I also put split loom on all my sensor wires so that they have a little protection from the flamethrower engine. I'm really amazed how hot this 4.0 gets and how hard it is to keep wires away from the hot spots.

jeepguy93

I replaced the entire Cps..

jeepguy93

Now it has a new fuel pressure regulator, fuel pump relay, distributor cap, rotor, wires and plugs, completely cleaned the I AC valve new TPs hey warrantied fuel pump so it's brand new, fuel filter...

In reverse it will run okay but and drive I think if you press the gas pedal it'll drop and get really close to 0 and died

jeepguy93

In park it'll idle no problem for ever, same in neutral and drive, but as soon as you press the gas it dies

jeepguy93

That's my idle

VTJeep

If its not the o2 sensor, I'd be surprised.

jeepguy93

Would there be a check engine light with that*?

VTJeep

Haha. You'd think so. There are plenty of times where it will store codes without tripping the CEL. I have a 99 with a similar issue. It will hesitate, buck chug and backfire at the manifold. At idle or partial throttle. At WOT it isn't an issue.
The symptoms all see to point to a fuel delivery issue, and as we've discovered, the o2's regularly show irregular voltage. I have a long list of replaced parts, including the o2's several times. Idk where the issue lies specifically as I haven't pinpointed/replaced the problematic part, but I'd suppose it lies somewhere within the o2's control..

Edit- oh yeah, it will do this when the codes have been checked/cleared and the CEL is off. Usually within 10 startup cycles, the CEL will return and the hesitation issue disappears and in back to a reliable XJ....

jeepguy93

So you really think it's an o2 sensor

VTJeep

Are you joking? My phone doesn't have the sarcasm update yet.. Just messin with you, of course!! No seriously, do as much diagnosing as possible, but yes I would suggest the o2 sensor.

jeepguy93

Well my brother in law just bought the Jeep and I'm helping him fix it . I have had 8 of these peeps and a couple of them have had similar problems so we are placed the parts for the previous owner had replace those parts so it's pretty much been narrowed down to the last thing we have is the o2 sensor! I can't think what else!

VTJeep

It's a process for sure. I've settled with allowing for a constant CEL and smooth running.. But not for lack of trying. I will suggest dropping the tank and inspecting it, as well as the filter. 97 and up has the in tank pump/filter assembly I believe. Mine had quite a bit of debris ranging from leafs to sand. Cleaning that out helped. Also check all grounds, and connections in general. Good vac lines, electronic components like fuses.. Check it all, I suppose.. Good luck!