I am considering buying another Cherokee as a project vehicle. It's a 91 cherokee 4x4 with the 4.0. The current owner blew the engine out and purchased a replacement 4.0 HO motor out of a 96 cherokee and dropped in the jeep, which will now not start. He claims that it's because the HO requires a higher-flow fuel pump. Is there any truth in this?

 

The HO does require a higher flow fuel pump than the '87 - '90 Renix motors. The '91 and '96 are, however, both HO motors. There may be issues with the electronics, because it's my understanding that sometime in '96 they changed from OBD1 to OBD2.

Maybe some else can elaborate, I just don't know about the specifics.

 

I believe that the 96 requires 49 psi of pressure, and the 91 requires 31.

So job #1 is to get a fuel pressure gauge and verify what pressure you have at the rail.

You'll need about 49, plus or minus 5 for it to fire.

 

Alright, I looked up the wikipedia articles on the XJ and I see where the 91-95 models had an HO motor as well, but in 96 they switched to a "Chrysler Block Cast" version of the 4.0 HO. Would this cause the kind of pressure issues that tjwalker was talking about? And would the OBD1/OBD2 switch cause the jeep not to start? And if that is the case how would you remedy that situation.

 

is the old motor still around

 

fuel pressure regulator creates fuel pressure pump creates flow no?

 

Just confirm the fuel pressure spec of 49 and if that is correct and you aren't there, that is your problem.

96 was kind of a hybrid year for fuel delivery. But I'm 99.9% confident that you MUST have 49 psi.

So you'll need the fuel pump system that gets you there.