Hard start, rough idlehelp diagnosing

[hear]

Yeah yeah, it’s an XJ. XJ forums aren’t nearly as helpful as this one normally is, and we have tons of underhood components in common so here goes.

Friends’ daughters 98 Cherokee has a really hard start & rough idle. If you ease up the throttle, it smoothed out nicely. If you stomp on the gas, it bogs down and dies or comes close to dying. You can see it running rough in the video, it shakes pretty bad at idle. At higher RPM it smooths right out.

Battery is good, took off the starter, cleaned it up & tested it, cleaned up a couple of grounds, same result. There was a TPS code but we replaced it. Code hasn’t returned (yet) but result is still the same.

Any ideas are welcome.

 

[Jezza]

Fuel pressure?

 

[Jerry Bransford]

How does it start when the engine is warmed up? Does the temperature gauge work normally?

 

[hear]

How does it start when the engine is warmed up? Does the temperature gauge work normally?

It might start slightly better if it's been warmed up, but nothing at all like any car I've ever had. I haven't had it running long enough to get up to temp (probably) and I didn't look at the temp gauge whenever I did let it run, but it does feel like it gets up to temp slowly. I didn't expect you to ask that, do you have a theory?

Since you asked, there is a leak in the upper passenger corner of the radiator. It'll shoot a little steam at you. They're on a school teacher budget, so it might be Bars Stop Leak on that for now, unless I can nab a radiator off a wrecked XJ.

 

[BIGMike]

From the video it sounds like it has a miss. Maybe try a compression test then you can narrow it to fuel or spark before throwing any money at it that would be in vein. Reading plugs might help point to which cylinder(s) has an issue as well. Also tracing and wiggling vacuum lines to find cracks or breaks can be useful and is free also.

 

[hear]

I'm confident there's a vacuum leak, just because it seems pretty likely. Haven't done fuel pressure test yet, but compression & plugs are on the list. A while back a shop we don't trust told her she had an exhaust manifold leak, but they opted to not do that work. I'm not sure how to check for that, nor do I have any idea what symptoms might be.

If there was a miss, shouldn't that throw a misfire code?

 

[TWiG01]

I’d start with the recommendations above for sure. Plugs, wires, vacuum. If you haven’t already, it’s definitely worth taking a look at the throttle body and air box. Far fetched, but Maybe a mouse has taken up residence and shoved it full of crap.

 

[hear]

Did a compression test, doesn't look good. I've never been down this road, not sure what my next diagnosis steps are.

Cylinder 1: 75
Cylinder 2: 75
Cylinder 3: 100
Cylinder 4: 15
Cylinder 5: 120
Cylinder 6: 30

 

[Jezza]

Did a compression test, doesn't look good. I've never been down this road, not sure what my next diagnosis steps are.

Cylinder 1: 75
Cylinder 2: 75
Cylinder 3: 100
Cylinder 4: 15
Cylinder 5: 120
Cylinder 6: 30

Did you try a wet compression test? If it were me I would do a leakdown next, and see where it is going.

 

[hear]

Did you try a wet compression test? If it were me I would do a leakdown next, and see where it is going.

Is a wet compression test where you drip oil through the spark plug hole, crank the engine to spread the oil, and then re-test compression? Because no, we didn't do that. I've never done a leak down test, I guess I get to research how to do that.

 

[Rickyd]

Is a wet compression test where you drip oil through the spark plug hole, crank the engine to spread the oil, and then re-test compression? Because no, we didn't do that. I've never done a leak down test, I guess I get to research how to do that.

Oil tests the rings to a degree. Leakdown will help you figure out which valves or rings are bad.basically listen for where its leaking.intake,exhaust or valve cover. Going off your initial comp reading the motor is roached though