Running Rich and Horrible Idle

[Nick91WD21]

Hello everyone! I just recently picked up a 1991 Pathfinder SE for hopes to make it into a daily/off-roader. The issue I’ve been stuck on solving is it’s rough idle and that it runs very rich (10 mpg).

The idling symptoms are:

-Struggles to maintain an idle and runs extremely rough. Usually jumps around 350-650 rpm

-It never cold starts and temp gauge never moves above maybe 1/8

-Sometimes idle will be so rough it causes it to die

-Also new plugs and 02 sensor have black fouling after only about 20 miles of driving

What I have replaced:

1. IACV & Connector

2. Air Assist Valve & Connector

3. Both coolant temp sensors & connectors

4. Spark plugs and wires

5. Distributor cap and rotor

6. 02 Sensor

7. Cleaned MAF and TB

8. Jiggled MAF connector and nothing changed (saw the ground wire is a common issue)

ECU codes:

Code 34 - Knock Sensor

(Unplugged the sensor and it still runs the same)

 

I feel like I’m running out of stuff to replace. What should I do? I am going to replace the thermostat next since even after both sensors and connectors were replaced the car doesn’t cold start and the gauge still always reads cold

[Slartibartfast]

Sounds like it's choking on fuel. The temp sensor would've been my first thought, too.

 

I don't think the knock sensor code is causing the problem. Mine had that code and seemed like it ran just fine. I think it got a little power back with the new sensor, but I wouldn't swear to that.

 

Are any plugs darker than the others? Does the oil smell like fuel? I'm wondering if you might have an injector leak. I would also check that the vacuum line from the pressure regulator to the intake isn't full of fuel.

 

Any idea what the previous owner did/had done to it? Hopefully it doesn't have the same junk injectors @peejay is working with.

[Nick91WD21]

Fuel system could definitely be an issue. I’ve never seen plugs look pitch black after 20 miles of driving. I checked them all and they all had pretty much equal fouling. Some maybe slightly worse, but all still black. Vacuum line to the FPR is clean and doesn’t smell. I’ll check if the oil smells like fuel tomorrow. What’s the best way to test for injector leak? 

[Nick91WD21]

Just checked and confirmed all the plugs have equal amounts of fouling. Oil does not smell like fuel, but I doubled checked the fpr vacuum line and it does have a slight smell of fuel. No liquid though.

[Slartibartfast]

The fuel smell is interesting. Might just be because there's fuel in the intake manifold. Just for grins, pull that vac line back off, and cycle the key a few times (to the run position, wait for the fuel pump to stop, then turn it back off). That should remove any doubt about whether the reg is leaking.

 

I could swear I'd found an injector leak test in the manual before, but I'm not seeing it now. Unfortunately there's no test port for a fuel pressure gauge, so you'll have to tee one in. Release the fuel pressure first (fuel pump fuse out, run the engine until it stalls, crank a couple times to be sure) so you don't spray gas from hell to breakfast when you open the system. Fuel pressure should be around 34 psi at idle with the vac line to the reg connected, 43 with it disconnected. Then shut it off and see how fast the needle falls. I've seen a couple of sources quote 10 PSI in ten seconds as the cutoff, but I'm not sure where they got that number.

 

With that said, if one injector was leaking, I would expect one black plug. I would not expect six bad injectors, unless the last guy replaced all six with the cheap crap ones. Six black plugs has me thinking the computer is injecting that much fuel based on bad info, which brings me back to ground you've already covered.

 

I know you've already done the coolant temp sensor, but I would go through the troubleshooting in the manual for that sensor/circuit anyway. Make sure the sensor is in spec, and make sure there isn't a wiring issue between it and the computer. Rule it out right proper.

 

On 3/3/2025 at 11:34 PM, Nick91WD21 said:

It never cold starts

 

I'm not quite following here. Hard starting, extended cranking? Does it start more easily when it's warm?

 

On 3/3/2025 at 11:34 PM, Nick91WD21 said:

8. Jiggled MAF connector and nothing changed (saw the ground wire is a common issue)

 

Might be worth throwing a new connector at it, on the off chance. Mine had a loose MAF connector, and surged/stalled at warm idle, especially stopped with the transmission in gear. I couldn't make it act up by jiggling the connector, but when it was acting up, jiggling it usually cleared it up. It's been fine since I replaced the connector (Rockauto had a replacement pigtail for it). Your symptoms don't sound like what mine was doing, though it sounds like it can manifest differently in some cases (see Precise1's second post in the thread linked below).

 

There was a plug-in harness with a ground lug to fix this issue. My donor car had one. I swapped it into mine, and to my surprise it ran like absolute crap until I took it back out. I still have no idea why. Possibly the loose connector fit worse in the repair harness than it did on the sensor itself? I decided I didn't need to poke that bear.

[Nick91WD21]

I’ll give that fuel pressure testing a try and see what info it gives. As for no cold start, what I meant is that no matter if it’s cold or warm it has the same exact idle symptom and idle speed. It doesn’t go through the normal cold start process of a high idle that drops down when it gets to temp. So I’m thinking the ecu isn’t getting the correct temp data. I’ll try wiring the sensor better. It has one power and one ground right?
 

Also I have a maf connector that I bought so I’ll give that a try as well.

[grygorie]

I had a similar problem on my old WD21, and it was a mixture of open stuck thermostat and tired gas pressure regulator. If the temperature remains low, the ECU thinks that the engine is still cold and enriches thoroughly. Replacing the thermostat seems like a good next step to me. Keep us posted, I'm interested!

You can post this as is on the forum. If you want a more technical or more formal version, I can offer you another one.