92 XJ, slow acceleration, possible vacuum leak?

My 92 XJ is having some problems accelerating. It will idle fine but when the engine is put under load there is a slight whistling when I tap the accelerator. I'm pretty sure this is a vacuum leak, but I'm thinking there might be something else wrong too.

I was just driving it on the highway and the acceleration on it was painstakingly slow. On flat land I can push in the gas and it will accelerate maybe 2 miles per hour, then it seems like it just runs out of power, several seconds later it will gain another few miles per hour. If I try and pull a hill to where the rpms drop below 2K it will begin to misfire.

I just put new plugs in it and checked the cap and rotor, they seemed fine. I recently replaced the fuel filter also. I don't think it's a fuel delivery issue since the problem is present throughout the entire RPM range. Essentially I'm wondering if a vacuum leak could cause the engine to act like this? And should I be looking in a specific vacuum loop, or just go ahead and do all new vacuum lines.

xj2win said:

My 92 XJ is having some problems accelerating. It will idle fine but when the engine is put under load there is a slight whistling when I tap the accelerator. I'm pretty sure this is a vacuum leak, but I'm thinking there might be something else wrong too.

I was just driving it on the highway and the acceleration on it was painstakingly slow. On flat land I can push in the gas and it will accelerate maybe 2 miles per hour, then it seems like it just runs out of power, several seconds later it will gain another few miles per hour. If I try and pull a hill to where the rpms drop below 2K it will begin to misfire.

I just put new plugs in it and checked the cap and rotor, they seemed fine. I recently replaced the fuel filter also. I don't think it's a fuel delivery issue since the problem is present throughout the entire RPM range. Essentially I'm wondering if a vacuum leak could cause the engine to act like this? And should I be looking in a specific vacuum loop, or just go ahead and do all new vacuum lines.

Click to expand...

Dumb question - does the problem go away or just lessen when the engine heats up? Also, have you verified function of your TPS?

The latter problem is usually found more in RENIX than HO, but the TPS has a good deal to do with fuel metering. It can be function-checked with an analogue (sweep needle) ohmmeter - you just disconnect the harness connector, grab two pins (typically the centre and one to either side - you're just looking for a variable resistance,) and watch the needle as you slowly and smoothly move the throttle by hand. The needle motion should follow the motion of the throttle blade (if you don't see any motion or any reading, twiddle the range knob until you do.) Absolute readings are not important here, what you're looking for is relative motion.

If the problem goes away when the engine heats up, torque the manifold screws. Pay particular attention to the ones at the rearward end of the engine, especially down bottom (I know, you really didn't want to hear that...) I find a 9/16" "flex" socket is a huge help here.

If the problem lessens after the engine heats up, it's probably also a manifold leak - but it's gone on long enough to burn through the gasket. Time to replace the gasket - I've covered that in fairly good detail a few times here before.

Very stupid question here: What about Air Filter?
Also, (novice here), but couldn't the IAC also possibly cause acceleration issues if it's problematic?

Hallo. The leaking manifold gasket is a possibilty and have to fixed when broken.
You renewed the plugs and cap and rotor, but do you have a good spark?
An ignition coil can also give problems. The idea of Jasorono about the air filter is also good.
Especially when the filter is wet by water. Don't trust it , but check.
Problems lately with the exhaust? The cat can be clogged.
The vacuum system contains a lot of connections. Wiggle and twist them to check, because sometimes they are only looking ok.
Plenty of work to do on your '92 XJ. LOL :clap:

greetings from an other '92 XJ

Yeah, if anything, the problem seems to get worse the longer I drive it, I'm assuming fuel is getting built up in the cylinders. I have a K&N on it and just recharged it when I did the cap and rotor. I recently cleaned the throttle body so I doubt the IAC is bad, I'm betting on the intake/exhaust gasket since the leak is probably somewhere between the MAP sensor and the cylinders.