Poor gas milage

I recently replaced my motor on a 89xj,i replaced with same motor and transfered all manifolds and sensors to new block, well im getting 10-11 mpg now and i used to get 16 and thats with the 31"s So 3 days after running new motor this idiot light comes on "service emission" and i thought its just a timer? but my question is can that have anything to do with it? I replaced o2 sensor about 10000 miles ago .. and i can smell it running rich. can it possible be the air intake sensor are have anything to do with coolant temp sensors? i did put a new regulator in cause to old was a leaker,but other that that it runs perfect, has its power back but milage is starting to suck a$$ any ideas? this is the last of the jeep project before i can say its "done". so i can find a new project..

I've been wrestling with the same problem.

I've cleaned out the throttle body - it was dirty, but it didn't help
I've run a couple of doses of seafoam through it - through the gas and through the manifold - didn't help.
I took apart, cleaned and reassembled all the electical connectors I could find - didn't help.
I've recalibrated the TPS, although it was fine - didn't help.
I've checked for manifold leaks - doesn't seem there are any.
I've replaced the O2 sensor - didn't help
I've replaced all the injectors - didn't help (anyone want a set of injectors?)
I've checked the MAP, which was fine although the manifold pressure is a *little* low.
I've checked all the temp sensors - didn't help.
I checked the brakes to see if any were binding - they weren't.
I checked the alignment - didn't help, although it *did* stop the jeep "wandering" along the road.

In the end I gave in and took it to a reputable independent garage, who discovered that the problem seems to be the computer - all the inputs are telling it it's running rich, but the computer thinks it's running lean... I've got a replacement computer (cost me $30 from eBay) on it's way, so we will see.

You might want to run through the same set of checks (search for MAP/MAT/CTS/TPS/...) and see if you can discover the problem (I wouldn't replace the injectors though, unless you actually find a problem - expensive!)

Was the computer reset after the new engine was installed. it can still be telling the new engine old information unhook the negitive battery cable and leave it over night ( you can do it for a minute but mine always work best over night ) and this will clear any information left from the old engine and reprogram itself to the new engine. If this does not work then start looking at other things. Was the fuel filter replace this is a common part that is cheap and is missed ALOT on regular maintenance till it's too clogged for the engine to start. Is it doing it all the time could of got a bad tank of fuel.I'm assuming the cap plugs and wire where done at the same time as the motor check to see if the plug gap is at spec it's .35 if you installed platnum plugs the don't work very well on the 4.0 I've run them but did not have the power or milage as the regular plugs. after that I'd be looking at the various sensers one could have cracked when moved to the new engine.

There is no reset on a Renix computer and they are pretty bullet proof. I think the MAP sensor is the main suspect for it running rich. The emission is a dumb timer. When it times out, the light comes on. Just unplug the timer and forget it. Make sure the MAP sensor is properly connected both electrically and the vacuum line. You can easily test the MAP sensor if you have a hand held vacuum pump.

Yes there is a reset or clear the codes or what ever you may want to call it but it's there if it's got a computer it's there.

The RENIX is a non-learning computer. It won't know the new engine from an old engine.

is there any solution to replace the computer, i have the same issue but i replaced the engine with a 4.2 so it runs like ass now. is there a computer that replaces te RENIX computer that can you can "learn"? thanks

rehab said:

is there any solution to replace the computer, i have the same issue but i replaced the engine with a 4.2 so it runs like ass now. is there a computer that replaces te RENIX computer that can you can "learn"? thanks

Click to expand...

What sensors are on it?

And what about the Crank Position Sensor? If that was somehow shifted it will run but the timing won't advance properly to burn the additional fuel dumped in during acceleration.

Always start at the battery, and look at the voltage sitting and running, and repair whatever is out of whack there, then address the ECM supply voltage that you actually measure at the ECM when running. Then Ohm out all the grounds and fix any defects you find in them. They mostly go to steel or aluminum so you may as well lift them and clean them because dissimilar metals begin to corrode at the moment of contact.

I could use a little help with mileage myself. My '87 gets 12-15 in town, actually barely 12 now in the winter and close to a solid 15 in the summer. OTOH all highway mileage nears 25 and I can't complain about that.

I think my low city mileage is mostly due to my short daily commute - one mile one way. So it probably never goes into closed loop. Still I'd like to know how to improve that and I'd be willing to bet spoofing the operating temp sensor in some way might help.

I don't know much about what goes on in the Renix ECM but I know it is rustic by today's standards and is more of a controller than a computer. I have a couple of PDF format FSMs but both were reduced too much when scanned and are tedious to read, so I haven't done all the investigation I eventually intend to do.

I'd bet though, that any of those that are getting only 10-12 MPG consistently are getting that because they are not going into closed loop, and I think just about any sensor out of range will cause that (and that would be as measured at the ECM, including the wiring and connectors).

So - troubleshoot all that and what's left is the less likely but still possible failure of a voltage regulator or input buffer within the ECU.

rehab said:

is there any solution to replace the computer, i have the same issue but i replaced the engine with a 4.2 so it runs like ass now. is there a computer that replaces te RENIX computer that can you can "learn"? thanks

Click to expand...

Yeah.

I'd think all the controller related parts from a Renix motor installed on a 4.2 would make the 4.2 work correctly off a Renix controller. Maybe an oversized throttle body and higher capacity nozzles.

If I were getting any more serious than that I think I'd look at repowering with something like a GM 4.3 or Ford 5.0 (and matching trannies) for which you can obtain (so I'm told) full breakout vehicle independent electronics package kits. Yeah it's not Jeep but the Re in Renix isn't such a great pedigree itself, and not much since then has been Jeep at all - just leftover parts the folks in Stuttgart were making them use up.

If I had a relatively clean and unblemished XJ (which I do) and thought it needed more power than standard bolt on upgrades offered, I'd shop the traders for a wrecked 4.3 Blazer or 1/2 ton, or 5.0 Mustang or Crown Vic. But then I have a large well equipped shop at my disposal which would make a conversion like that much easier.

And I do like to make noise & smoke tires, but have no particular desire to do that in a Jeep. Right now outside sits a 454/TH400 Suburban waiting to become a donor for a noisemaking / tire smoking project. Haven't found the right project car yet, but would like a nice '67 to '72 Chevy half-ton, or barring that something one wouldn't expect to find a big block in.