my 1996 4.0 Sport has an annoying idle issue. It's not driven much and I'm getting it ready to sell.

it always starts fine, sometimes with a little timing chain rattle (do they stretch?)

It wil either: idle ok ish, a few skips here and there, as it warms up the idle goes up. Around 2k RPM. This leads to problems at a stop as the car wants to take of.

The other issue is that at cruise speeds and at quarter or less throttle (just maintaining speed) the car lurches back and forth subtly but noticeably. I believe it's a misfire of sorts. In the higher RPM's it pulls strong.

When I turn it off after running for a bit sometimes it will idle very very low. Around 400 rpm. It doesn't seem to have any consistency to it. It's just annoying. I've replaced the IAC. No difference.

It also has a delay of the line, give it gas and it takes about half a second to respond. Throttle response otherwise while driving is good. the car accelerates well and is fast.

High idle and skipping while cruising both need to be fixed.

Ideas?

 

 

Something brought up by someone else was the ground strap between the starter bolt to the car frame . Without getting into the logic of the effect this has on the engine regulation it's a free try . Under the heading that ground straps are as important as the power or intelligence signal wires or functionality of the attached devices is the grounds are just as important . The large strap is almost impossible to get to from the top but is easy from the bottom with a 13 mm and 8 or 10 mm for the car frame .Be sure to mechanically clean the body / frame surface .

Throttle return spring strong enough to bring the butterfly closed ? There is a TSB on replacing with a stronger spring or others have done a cheaper DIY modification .

http://www.jagrepair.com/images/TSB/...igh%20Idle.pdf

A cheap try with the timing chain is to loosen up the sliding tensioner mechanism with some oil treatment as they bind and not expand with oil hydraulic pressure hence a rattle in the chain . There is a oil viscosity issue as well . There is a brand of oil filter that collapses internally that reduces the oil pressure to expand them in a timely manor after start up . There is also a TSB on a improved tensioner .

http://www.jagrepair.com/images/TSB/...er%20Start.pdf

 

This got me by surprise the other day, might be worth a look as well.

 

So much overthinking here, misfire is a classic failing coil/coils, especially when the rpm goes up and down while cruising. Dont ask how much for new genuine coils that seem to be the only ones that will work longer than 2 years
Sluggish start will be due to either or both, stretched throttle cable and therefore TPS not working in full range or maybe a cable mounting bush which often splits and before it engages the pedal it has to go through split rubber, hence half a second lag.
TPS works in 0.5 to 5V range, with stretched cable mine was working from 0.5V to ~4.2V at max open throttle. As you can imagine it got to a point where it felt like I'm running a 4 cylinder car, adjusted line to open to ~4.9V and it felt like I've added a turbo.
When you adjust throttle cable I bet the throttle wont close fully so either follow DIY modification mentioned above or simply add an extra turn to each of the three springs in the throttle body, it will then close much easier.
There might be other reasons too like air leak, leaky or stuck injector, fuel pressure issue, etc but at this point coils are always a number one issue with idle with these cars.