1990 4.0 cherokee how do i change from fuel injection to carb
- Thread starter ZZMAN
- Start date
greg92jeepxj
Official Supporter
I haven't heard of it being done , so I don't know how It would work out , but a 4.2 (258 ) intake is supposed to bolt up however port matching could be an issue. Compare intake/exhaust gaskets for 4.0 and 4.2 before attempting . Don't want to shoot you down from such a project as I can understand why , but consider advantage versus disadvantage first. Yeah , I like carbs for simplicity of rebuild and tuning , but injection got that beat by a mile . Cheaper to rebuild a carb than buy new injectors , probably what your jeep needs , maybe an electric pump too ( more money ) but once running well , fuel injector cleaner and regular filter change should keep it going for a long time . Maybe an occasional in-line filter change (inside of tank) is clogged and troubling your jeep too. Fortunately the cherokee XJ sending unit on the tank is easy to get at without tank removal , just get jeep on jack stands with wheels off pavement and rear axle drops low enough to get to sending unit to remove it for pump and filter sock change. If fuel system is in shape , sensors and wiring harness ok , PCM replacement is all if no issues with ground wires . Don't want to test your patience as the jeep is already doing that , but this whole prospect has me making a list of questions ; fuel pump pressure ? Injector condition ? Sensors (O2, MAP, etc. , etc.) ? Engine wiring harness ? Troubles codes ? Injector harness ? EGR ? I can go on , but my point is , before considering the conversion to carb , make certain your problem is not outside of fuel system . Imagine swapping intake and carb, adding a fuel pressure regulator for carb fuel line , swapping a PCM for a carbureted system not injected which will use the dura spark module and distributor , this is all off the top of my head. But how much will you spend for the correct parts to make it run correctly ? Even from salvage , still spending money that could go into a good set of injectors if that is all that is wrong. A noid light to check injector harness , fuel pressure gauge , maybe a scan tool to read O2 cross counts , digital multimeter is a good start.
Dont want to talk you out of it and you probably already checked these things , but we still have to tend with inspection and emission and you don't want the hassle of equipment failure on inspection . But if your dead set on it , intake and carb from 258 , throttle and TV cables and brackets , in-line fuel pressure regulator, distributor and dura spark compatible with PCM for carb set up( cannot use same PCM for fuel injection) PCM and engine harness , should be good for start. You may find more item in need for swap. Must check for compatibility , I.E. , will power steering pump bolt up to 258 intake ? Will exhaust manifold flange clear 258 intake ? I may be missing some items , but i don't have a 258 to compare. I suggest finding the parts for comparison to make certain they will bolt up . You don't want to start this job and find out in the middle you cannot finish such as reinstalling power steering pump which bolts to intake. Please compare carefully and consider the swap versus fixing your existing system. Check sensor comparably with swapped PCM , a parts store can verify if sensors are different by looking up year or check on line the same way. Basically , 258 intake and Renix distributor , PCM and ignition system to start with . Best of luck , the decision alone will be the deciding factor after researching necessary swap parts.
Dont want to talk you out of it and you probably already checked these things , but we still have to tend with inspection and emission and you don't want the hassle of equipment failure on inspection . But if your dead set on it , intake and carb from 258 , throttle and TV cables and brackets , in-line fuel pressure regulator, distributor and dura spark compatible with PCM for carb set up( cannot use same PCM for fuel injection) PCM and engine harness , should be good for start. You may find more item in need for swap. Must check for compatibility , I.E. , will power steering pump bolt up to 258 intake ? Will exhaust manifold flange clear 258 intake ? I may be missing some items , but i don't have a 258 to compare. I suggest finding the parts for comparison to make certain they will bolt up . You don't want to start this job and find out in the middle you cannot finish such as reinstalling power steering pump which bolts to intake. Please compare carefully and consider the swap versus fixing your existing system. Check sensor comparably with swapped PCM , a parts store can verify if sensors are different by looking up year or check on line the same way. Basically , 258 intake and Renix distributor , PCM and ignition system to start with . Best of luck , the decision alone will be the deciding factor after researching necessary swap parts.
Last edited:
MudderChuck
New member
I'd go down to Home Depot and buy a $3ß volt ohm meter, Test the CPS for ohms. my guess is it will read around 180 ohms or higher and you need a new CPS. If the ohms test out OK, scrub the connector pins with a good contact cleaner. Fish out the wires from the CPS to the connector and make sure they aren't burning on the exhaust manifold.
You can also volt test the CPS, turn your meter to the lowest volt scale amd stick the lprobes into the disconnected (and cleaned) CPS connector and have someone turn the motor over, you should get a 0.5 volt pulse or more, any less than 0.5 volts and you have a weak pulse.
My 88 did just about what you describe, it got worse as the motor heated up, the CPS ohms (resistance) went up as the CPS heated up. It can be something else, but this is a starting point.
An old analogue meter (with the needle instead of a digital readout) $7.00 actually works better for this test as long as it has a low enough DC volt scale.
My old YJ had a carb, it was a bigger pain in the arse than my Renix ever was.
If my hunch about the CPS doesn't work out for you, I have some other ideas, less likely but possible.
You can also volt test the CPS, turn your meter to the lowest volt scale amd stick the lprobes into the disconnected (and cleaned) CPS connector and have someone turn the motor over, you should get a 0.5 volt pulse or more, any less than 0.5 volts and you have a weak pulse.
My 88 did just about what you describe, it got worse as the motor heated up, the CPS ohms (resistance) went up as the CPS heated up. It can be something else, but this is a starting point.
An old analogue meter (with the needle instead of a digital readout) $7.00 actually works better for this test as long as it has a low enough DC volt scale.
My old YJ had a carb, it was a bigger pain in the arse than my Renix ever was.
If my hunch about the CPS doesn't work out for you, I have some other ideas, less likely but possible.
greg92jeepxj
Official Supporter
Not willing to just post and leave it at that , I further researched your post history and found that your problem dates back to , APRIL !!! This upset me as I am now reminded of my own PCM problem I had back in 2004. My jeep , a 1992 , cannot compare with your 1990 since mine is a Chrysler and yours is an AMC. My PCM warranted exchange because I accidentally spiked it testing circuits from relays WITHOUT a computer safe test light. My bad. Enough of me , lets look at your situation . I tried to google the PCM #'s 8953005428(your original) and 8953005225(swap) and found Rockauto lists them in the same application . We can only say they should both be applicable to your jeep . Still asking why did your first PCM take a dive , but lets move on. If your satisfied wiring is not shorted or bad in any way , that leaves the alternator and ignition. My Chrysler PCM internally fires ignition coil and controls spark advance as well.
your AMC has dura spark , correct ? Since the jeep is starting and running , albeit poor , this leaves to make certain of alternator , wiring and field , to battery and PDC . Is PCM getting enough voltage ? We need to know . But equally important and possibly MORE , the dura spark . At this time , I do not feel the PCM is responsible for the jeeps loss of power but am leaning toward the ignition module. I cannot help but question WHY the original PCM failed , this is ALWAYS important in diagnostics but if you feel all is well with engine wiring harness and no shorts or otherwise issues with wiring , ign. Module is probable cause . I know you have suffered endless torment , been there , but this is worth checking since nothing else produced results .
it starts and idles but won't accelerate . If fuel pressure is ok as you previously posted , I believe , spark advance is the problem . The way you described it in your first posts was like NO ADVANCE , running like constantly retarded and stalling , on acceration . THIS SPELLS WEAK SPARK TO ME and 1990 AMC should have the motorcraft dura spark system . If not in the module itself , the distributor pick up . But I'm now believing its ignition . Your PCM seems to be working and I do not see a reason to fault it yet. But not to test your patience any longer , do you know why the original PCM MAY have failed ? You know this is important . Best of luck , wishing for success and hope for your post soonest. Greg
