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Tps or IAC or ???

2

2jeepdrivers

NAXJA Forum User
Location
Rogersville,mo
Took my beater XJ wheeling saturday and halfway up a ledge on the way back to the truck it just died. It will restart but it idles real rough and will not rev. I did some searching thinking it was the TPS but it really doesn't sound like that is what it is, everyone else I found had symptoms before hand and mine didn't it was running great until it just died. I pulled the idle air control because I noticed the bolts were loose and it is pretty crudded up but I didn't know for sure if that could be the problem any other vehicle I have had that the IAC went bad in it would idle real high and then come back down. So for all of you 4.0 gurus what does this sound like it is a 92' 4.0 with an auto behind it.:helpme:
 
Sounds to me like the vacuum hose from the throttle body to the MAP sensor came off. The MAP sensor is connected to the firewall right beside the valve cover.
 
My MAP connection came out at the T/B, jeep ran like crap & died untill I found it, I dont know how it dod it but it just popped out.
 
Checked that and the hose is fine but if I unplug it and just block off the intake vacuum port and block off the line it idles smooth but still will not rev. I took off the idle air control and cleaned it all out but that didn't seem to effect it. (edit) I just realized that it was said that the vacuum line runs to the throttle body off of the map sensor, mine is going to the intake would that throw it off.
 
Last edited:
Alright I have put a new vacuum line on the map sensor. Checked voltage on the throttle position sensor, cleaned the IAC and still starts and idles but dies after about 30 seconds will not rev. I have not found the exact spec on ohm readings for a CPS but if I ohm any of the wires I get that they are open but I thought if it was completely junk it wouldn't start. Can somebody please give me some feedback on this I want to go wheeling tomorrow.
 
ehall said:
Oil pressure switch to the fuel pump relay maybe

When you turn the key on, the pump primes up, but from then on the oil pressure switch drives the pump
Click to expand...

OPS to fuel pump? On a 92? How does it ever start if that is the case? You lost me?

It sounds like there is no TPS signal to the ECU, or its a bad MAP signal to the ECU, or something like a bad high voltage wire grounding out between the cap and HV coil (pop the hood and crank it in the dark and watch for arcing), or a bad fuel pressure regulator, bad fuel pump (get a gauge and test the fuel pressure at the rail port), or bad ballast resistor (ohm meter test if 92 had them?) on the power to the fuel pump, or just maybe the CPS is dying.

The MAP sensor vacuum line is supposed to be attached to a vacuum port on the side of the throttle body, not on the intake manifold. Sounds like someone did some custom work there, so I don't know what to tell you.
 
ehall said:
There are two relays--one to prime the pump when the key is turned on, and another that is driven by the oil pressure switch
Click to expand...

Wow, that's a new one for me. Was it introduced in 91, OBD-I?

Is it two relays, or two different paths to the relay?

I wonder why they did not just use the ECU and one relay, and let the ECU monitor oil pressure????

Can you post a drawing of it?
 
Alright I have chased all of my grounds they all seem alright and they ohm at zero or real close. On the map sensor where is the port on the throttle body that the vacuum line goes to, I have it off and there seems to be nowhere for it to hook to. I am leaning towards a throttle position sensor now because I can get it to start on a regular basis but just won't rev or idle for very long. I am getting 52psi at the fuel rail at idle so I don't think it is a fuel delivery problem. Already tried looking for arcs found a couple bad plug wires so I replaced all of them but that didn't change anything.I am going over to my buddy's to steal his throttle body with all the sensors still intact so I will see if I can narrow it down.
 
2jeepdrivers said:
Alright I have chased all of my grounds they all seem alright and they ohm at zero or real close. On the map sensor where is the port on the throttle body that the vacuum line goes to, I have it off and there seems to be nowhere for it to hook to. I am leaning towards a throttle position sensor now because I can get it to start on a regular basis but just won't rev or idle for very long. I am getting 52psi at the fuel rail at idle so I don't think it is a fuel delivery problem. Already tried looking for arcs found a couple bad plug wires so I replaced all of them but that didn't change anything.I am going over to my buddy's to steal his throttle body with all the sensors still intact so I will see if I can narrow it down.
Click to expand...

Somebody correct me here, but isn't the fuel pressure 39 psi with the vacuum line disconnected and 31 psi with the vac line connected on the fuel regulator which is on the fuel rail. I know they raised the pressure in later years, but I was thinking it was OBD-II 96 or later. He has 92, HO.

THere are two holes side by side on the lower side of the throttle body, one is a blind hole the other has port vacuum.
 
Something you haven't thought of.......plugged CAT.
 
Ecomike said:
Somebody correct me here, but isn't the fuel pressure 39 psi with the vacuum line disconnected and 31 psi with the vac line connected on the fuel regulator which is on the fuel rail. I know they raised the pressure in later years, but I was thinking it was OBD-II 96 or later. He has 92, HO.
Click to expand...
That is what the 91 FSM states
 
Ecomike said:
Is it two relays, or two different paths to the relay?

I wonder why they did not just use the ECU and one relay, and let the ECU monitor oil pressure????
Click to expand...
I got confused with my cars--GM runs the fuel pump power through the oil pressure switch so that the fuel pump shuts off if the engine dies (as in an accident). According to the 91 FSM there is one relay, and run-time is controlled by the ECM:

The engine controller energizes the fuel pump through the fuel pump relay. Battery voltage is applied to the relay from the ignition switch. The relay is energized when a ground is provided by the engine controller. The relay is located in the power distribution center next to the coolant recovery bottle (Fig. 3).

So the key activates power, and the ECU grounds the pump. However you can hear that the pump is primed before startup just by turning the key to ACC position, and you will hear the pump engage enough to establish line pressure.

As for the OP, the theory still stands more or less--it could be that the fuel pump is cutting off after the engine starts because it is only running on the fuel that was delivered by the priming operation. I suspect this because (1) he has no throttle and (2) the engine dies a few seconds after starting, both of which indicate no fuel delivery after startup.
 
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ehall said:
That is what the 91 FSM states
Click to expand...

In that case he has documented that his fuel pressure is too high, 51 psi, so it may be flooding out. Sounds like the fuel pressure regulator has clogged up, and gone bad.
 
Just put an adjustable regulator I had on it to knock the fuel down. My throttle body doesn't have any ports like you were talking about. Also I don't have a cat so it can't be plugged.
 
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