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4.0 Eng swap now runs rough

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Hey guys,

I've recently swapped a perfectly running engine for my 88 MJ(manual) to my 90 XJ(Auto) and now the xj is not running right.

I only swapped the eng, belt accesories, intake, fuel rail and injectors. I reused the TB and all wiring from the 90 XJ.

The XJ will take about 3 sec to start and then idle at 1000rpms then drop down to 700rpms. It can sit and idle with no problems. Once it hit the gas it will rev up and come back down as i let my foot off the gas, but it will drop down to a lower rpm and sputter. The higher i rev it the lower it will drop down afterwards and sometimes die. I can press the gas and keep it going.


The xj now has new plugs, wires, cap, rotor button, cat+muffler.

I've cleaned the TB, IAC, check and adjusted the TPS. 4.7in, 4.2out


Any suggestions? Thanks


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to me it sounds like a fuel issue


That was my first guess. I replaced fuel filter and tryed bypassing the fuel pump resister but it still acts the same.


It is harder to start once it dies. It will spin over and sputter for alittle while then it will run fine.


Thanks for the ideas, keep um comming please.


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It was the TPS, however, I didn't need to replace it.

I decided to pull it off and check the resistance between the input and output while moving the throttle. It was operating correctly, the resistance would drop as i opened the TB. It turns out the TPS on the TB that I got from my donor engin wasn't installed right. The lever on the TB wasn't moving the lever on the TPS. That's why my output at idle and WOT were the same, the TPS didn't know the TB was opening.


I love the problems that you don't have to spend money on to fix. jamminz.gif

Thanks to everyone who helped me figure this out.


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