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Old 03-22-2022, 06:23 PM   #1
PaulC
 
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Default Running rich issue (not driveable barely idles)

So I've been working on this issue since I purchased the truck. It's a 93 with a motor from a 95. It came from a salvage yard and was missing the fuel rail and injectors. Once those were replaced I found this issue with the truck barely able to idle and it's very rich to the point that it has gas dripping from the exhaust. It will only idle with the air filter removed as soon as you put it on it dies and even with it off you may have to give it a little throttle to keep it idling. When I unplug the MAF it idles perfectly. Here's what I have done so far. I replaced the fuel pressure regulator which was indeed bad (no change). I have replaced the MAF 3 times, the last one being a brand new OEM Hitachi (no change). Reading the voltage from the MAF sensor while idling it is around 1.7 volts which is within spec. I have checked the fuel return line and it is clear. Checking the TPS i see .47 volts at idle and it goes up when the throttle is moved. I did find that my EGT sensor is bad as it shows completely open when checking resistance across it. I don't think that could cause this but let me know if I'm wrong. I've disassembled the throttle body and cleaned everything. I have also checked for vacuum leaks and so far haven't found any. The only code it's showing is for the IAT sensor which I know is broken, but don't think would cause this issue (let me know if I'm wrong). I have a vacuum/fuel pressure tester coming thursday so I can confirm everything is good there. Talking to a couple mechanics they suggest going ahead and replacing all the sensors. One said he ran into a similar issue on a hardbody and after all sensors were replaced it fixed it. It's also been suggested it could be the ECM. If anyone has ideas on what I should check next or troubleshooting tips please let me know.
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Old 03-22-2022, 09:05 PM   #2
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Have you checked timing? IAT will not cause this issue. Are you sure the injectors aren't sticking open?
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Old 03-22-2022, 09:11 PM   #3
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Since it idles perfectly with the MAF unplugged I believe that eliminates either of those issues. Would that be correct?
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Old 03-23-2022, 12:49 AM   #4
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Maybe I missed it, but have you tried starting fluid for a vacuum leak on the hoses/lines and intake?
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Old 03-23-2022, 01:40 AM   #5
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Yep, I did check the vacuum lines and all around the intake and throttle body for leaks with starting fluid. I have a vacuum/fuel pressure gauge arriving Thursday so I'll know for sure then if either are issues.
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Old 03-23-2022, 06:44 AM   #6
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I don’t like the notion of “replacing ALL of the sensors” before it could idle and run correctly.

How would you know what actually fixed the problem?! How much would that even cost? Not practical and sounds fishy. Apologies if you trust this mechanic that gave that advice. I don’t buy into it.

There seems to be unregulated fuel flow, if the truck is running seriously rich and fuel is dripping out of the tailpipe. Working from the fuel circuit, the fuel pump pumps fuel to filter, to the fuel rail, fuel pressure regulator, and then to the injectors. Are you certain the FPR is working correctly? What is the Resistance across the 2 pins of each injector? What is the fuel pressure at start up? I also wonder if there is a misfire happening with all of the fuel pouring out the tailpipe. Can you confirm proper spark for each cylinder?
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Old 03-23-2022, 08:27 AM   #7
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I'll be able to confirm fuel pressure tomorrow. I have checked resistance across each injector and they were all exactly the same and within spec though I don't remember what that was at the moment. Whenever it idles while the MAF is unplugged they are all firing. It seems to me like when the ecm is in default mode it's idling great which would mean everything outside of sensors are working. As soon as the MAF is plugged in and the ecm begins relying on sensor data to determine how much fuel to deliver it just starts dumping fuel. So either the ecm is bad or a sensor is bad. That's my thought process at least. Let me know if that makes sense.
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Old 03-23-2022, 02:09 PM   #8
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Are you getting any codes?
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Old 03-23-2022, 02:30 PM   #9
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The only code is for the IAT. Which I know the wires are broken on.
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Old 03-23-2022, 05:34 PM   #10
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In tracing out the circuits for the 1995 KA24E motor. I started at the MAF sensor. One leg goes to Ground(make certain your Grounding points are clean and tight AND tested with a meter). The other leg goes to the “Check Connector”(I don’t know what that is) and then connects up with the solenoids of the EGRC, PAIRC and SCV(Swirl Control Valve) and what I feel is the most important component, the IACC-AAC valve(Idle Air Control C(?)-Auxiliary Air Control valve). This last component controls the idle air flow and uses a plunger that gets carbon build up and won’t allow it to operate properly if not maintained. It will increase idle air flow, say if the a/c is turned on, it will energize and allow more metered air flow to increase the intake air so that the engine does not stall out due to the increased load on the engine. The solenoid valves allow vacuum to be stopped or allowed to flow depending on when it receives an electrical signal from the ECM.

The first leg of the MAF sensor is also connected directly to your O2 sensor and then to Ground. So this might be worth checking into to eliminate some circuits/components to narrow down the issue(s).

I’m also thinking about what is different from the ‘93 motor to the ‘95 motor, if any. Did Nissan make any internal/external/ECU programming changes between these years? I only know that they are OBDI Compliant. This question could be a non-issue. But just thinking out loud to help.
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