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HELP!,Problems with 2.5L

O

oldmopars

NAXJA Forum User
Location
Roy Washington
I am still having trouble with my 2.5L Comanche.
The problem is that the engine will rev ok, but not under a load. If you try to accelerate the engine pops through the throttle body.
So far I have replaced the Plugs, wires, cap, rotor, Coolant temp sencor, and O2 sencor.
I also swapped out the Coil with a good one and the Manifold air temp sencor(?).
I checked the timing and it seemed OK, but the sticker on the hood said its not adjustable. It was at about 10 deg.
What now? I am getting tired of throwing money at this thing.
Please help, any suggestions would help.
 
Just throwing this out there, but maybe some carb cleaner in the throttle body might do the trick. Its a long shot. fuel injection engines have alot of sensors, MAP, O2, sensors in the throttle body itself. If any of those fail that can mess you up preety bad. I'd atleast double check all those connections.Even though you replaced the o2 it could have been a bad replacement piece, just a thought.
 
Also you can do an engine diagnostic yourself.Your check engine light will blink a certain number of times after you turn your ignition on/off 3 timmes and that will be your problem.
 
Diablito said:
Also you can do an engine diagnostic yourself.Your check engine light will blink a certain number of times after you turn your ignition on/off 3 timmes and that will be your problem.
Click to expand...

Not if he has an '86, '87, '88, '89 or '90. Those years were Renix-based and do not return trouble codes. In fact, they don't even have a trouble light. The "Maintenance Required" light is just a simple tomer to remind you when to replace the oxygen sensor.
 
Have you checked the fuel pressure? On those throttle body units you have to buy an adapter with a shaeder valve in it. There is a hex plug on the drivers side rear of the top of the throttle body you remove, and screw in the adapter. Have you checked the distributor shaft bushings? If the shaft is wollowing, the timing could vary greatly. On my 86 2.5l, the hose from the map sensor was cracked and caused some drivability problems.
 
I have not checked fuel pressure, I don't have the equiptment to do that, but it may be time to buy it. I did check the distributor and it was tight.
I will try the carb cleaner, its cheap and can't hurt.
What do you all think about it maybe being the Fuel filter. I just realized I have not checked that.
It does seem, however that it is ignition related.
Anything else that you can think of would help. I an going to work on it today.
 
I vote for fuel delivery, bad pump or clogged filter. If it revs ok but dies under load then it sound like not enough fuel. Put the adapter in and slap on a pressure gauge.
 
I replaced the fuel filter and checked the TPS and MAP sencors. I do not own a pressure guage.
It does rev, but it will pop if you try to rev it fast even not under a load. Like if you try to rev it up fast while just sitting, it will still pop. If you drive under a load it will also pop.
I can see it blow flames out the TB boot.
I am at a total loss, I do not know what else to check. I guess I have no choise but to buy a guage and adapter.
What should the pressure be?
 
Low fuel pressure will cause the syptoms you give. Will make you think your timing is retarded due to backfire when you open the throttle. Just experienced this on my wife's 89 4.0 Renix.
 
I have a AMC/Mopar Throttle Body fuel injection manual for 86 and 87 model years and it calls for 14.5psi with the engine at 2000rpm. The fuel pressure regulator is part of the TBI assembly and is adjustable with a torx type scew driver. The adapter is part number 8983 501 572. I ordered mine from a Jeep deeler, it was only a few bucks, how ever I was unable to get the hex plug out to install it. If you have an AC volt meter, you might unhook the CPS(crank position sensor) and make sure you're getting a good signal. The CPS is what triggers the ignition control module that saturates the coil. I'm not sure what the spec for the 2.5 is but for the 4.0 it should put out a minimum of 5v AC.
 
I was looking through my old 84-86 shop manual troubleshooting guide and came up with the following:1) firing order is shown wrong and a bullitan corrects the incorrect diagram, firing order is 1-3-4-2, rotating clockwise looking down on distributor 2) trigger wheel pin missing, it does not define trigger wheel but I think it is refering to the flywheel and CPS 3) two plug wires of consecutive firing cylinders routed next to each other, re-rout plug wires away from each other.
 
Ron Hyslop said:
2) trigger wheel pin missing, it does not define trigger wheel but I think it is refering to the flywheel and CPS
Click to expand...

Probably not, unless you were looking at an addendum in the back of the book. The '84 and '85 4-cyl were carbureted with a transistor ignition, and did not use a CPS. The '88 FSM is actually an '85 FSM with some TSBs added to the back, and a separate manual available to cover the '86 TBI, so I assume the '86 FSM would be the same. The "trigger wheel" they refer to is probably the magnetic coil in the distributor that excites the ignition pulse. That is a separate part available for field service in the '84 and '85 4-cyl distributors.
 
My big yellow shop manual has sections on both carberated and TBI for 4 cyld motors(and diesel stuff). It has service bullitans bound in the back including supplements for the 87 4.0l stuff and another for 88 4.0l stuff.
 
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