Ever find your problem?

 

Thanks for checking in, reboot. I haven’t solved it just yet, life got in the way this past week. I still plan to tackle removing and replacing the CKP and wiring. Currently waiting on an 18” socket extension to help reach the bolts without (hopefully) dropping the exhaust out for access.

I think I was able to rule-out the VSS, took it for a short drive the other day with it unplugged and sure enough, about 15-20 minutes after getting off the highway, engine bay temp went up, the gremlin woke up and the poor running condition started again. I feel like that leaves just the Crank or Cam position sensor assuming it’s not something more complicated. Will check back in as soon as I service that sensor.

 

Made a little progress today: picked up an 18” socket extension (as recommended in another thread) which made the job of reaching and removing the sensor cover much easier.

I was looking for a plug further up the engine block like I’ve seen in others’ posts but in my case (4.0 GEMS V8) there is no second plug, the wire goes up the firewall and disappears into a harness by the fuel rail.

I found it very hard to reach the sensor plug to release it, but by gently pulling up on the harness from behind the engine I was able to lift it all up and nudge it into a better spot where I pulled the old sensor from the cover and unplugged the wire.

I peeled back the wire cover, which was destroyed by heat & age and the insulation on the wires had also been damaged.




I cut the wire leaving a few inches on either side and brought everything inside to clean & repair.

The cover and the old sensor are intact, just filthy and cooked well done. The sensor cover has some fiberglass insulation stuck to it, I didn’t bother scrubbing all of it off.




I wanted to replace as much of the wire harness as I could, but I couldn’t find a replacement plug or a way to remove the wires from the plug (I’ve read there are identical plugs elsewhere on the vehicle, including the fog lights) but I decided to re-use what I had.

I cleaned everything thoroughly with a gentle degreaser and then electric contact cleaner, then added some thick heat-shrink insulation, followed by some spade connectors for a durable connection back into the original harness and a layer of heat-shielding made for engine bay wiring.




Tomorrow morning I’ll go install the repaired wiring and a new sensor and another layer of thermal protection. I’ll try to coax the gremlins out and report back. On one hand I’m worried that my repair might only amount to an expensive and time-consuming bandage, but I think if heat and poor insulation is the cause of the problem, this should be a solid fix.

 

if you ever need plugs:

https://www.ebay.com/itm/131030682284

https://www.ebay.com/itm/141206273862



hope you got it repaired

 

It’s a sad day when the work doesn’t pay off!
I finished up splicing the sensor plug back in, reinstalled the sensor and cover which was actually very easy: I plugged in the sensor and put it in a tiny plastic bag to keep it clean before dangling it back into place with the wire. I was able to install the cover and the bolts onto the sensor and finesse it into place on the mount. With my other hand, I got the two fixing bolts started and in a few moments I was done!

Hooked up the battery terminal again, and she fired right up, no noticeable difference from the old sensor, not that I expected any.

While I was waiting for the engine to warm up, I spotted a new transmission fluid leak that I was able to patch up with self-fusing silicone tape and a fiberglass patch (just temporary until I get the line replaced. I highly recommend the silicone tape in anyone’s emergency kit for ruptured hoses or as a heat-resistant alternative to a zip-tie.

After sorting out the leak I fired it up again and took it for a test run, I know from experience with this gremlin that it takes about 30-minutes of slow driving to get it to show up. I switched my UG to display the MAF voltage, timing advance, engine temp and air intake temp.

Exactly 30 minutes later a traffic light turns green and I give it the gas and poof! It stalls out dead instantly. Started back up and idle-crawled off the road. My engine temp at this moment is 141f, coolant reads 194f, MAF is normal (0.08 at idle) and timing advance value reads 28-30, I give it some more gas and the timing advance drops to 10-14 and it starts to stall and sputter. I also noticed that the MAF reads extremely low while it’s sputtering, it starts to show values down to 0.01, way below the idle rate, despite having the throttle open. t should be reading 0.20-0.30+ with a bit of throttle applied.

I limped home, luckily I wasn’t even 500ft away! I took a video so hopefully you all can get a clear view of what’s going on as I see it. Starts up and idles happily. Pushes 30lbs of fuel although with my cheap tire-pressure gauge I noticed it doesn’t build fuel pressure under heavy throttle, it holds and 30 and sometimes drops below. I even noticed the fuel pressure goes up when it starts spitting.

At this point I’m frustrated, but I know it’s not the VSS, very likely not the Crank Position Sensor unless my repair was just inadequate, could be a fuel pump, although it seems totally related to runtime and/or engine bay temps, so it could be a break in the harness elsewhere or the Cam Position Sensor. The only two data points that go weird in this condition are the MAF and Timing Advance values, everything else seems normal, including throttle position and vehicle speed.

What do you folks think? Where should I look next? The MAF is brand new, and this also happened both with the original and no MAF sensor connection.

 

I posted this same question on Reddit and someone recommended I check my idle air control valve. I’m going to look into that next since it’s easily accessible. I’ll post any findings here too.

 

Did you say you installed a new CKP sensor? Just because the insulation is cracked up doesn't mean the circuit is shorted, so the repair work might not have changed anything. Around $50 for a nice new NGK sensor, that's only half a tank of gas...costs even less other places.

IACV controls idle, so if idle is good, then IACV is working, but good to be cleaned up anyway, along with passages and throttle plate. Check throttle plate is exactly at center rotation while you are there (distance to top and bottom equal). If bottom is shorter, that could cause hesitation when gas pedal is pressed. Be aware if the TPS is moved, then the adaptive values might need to be reset, otherwise really high idle when started.

Hope this helps.

 

Thanks for weighing in, I did replace with a new sensor, just because it was so cheap and I was already removing it to inspect the wires.
The idle has always been good, I had ruled out the IACV as most people agree that if it’s malfunctioning it will affect the idle. I have done a throttle-body cleaning just with seafoam just a few weeks ago. Same problem existing before & after.
Someone who saw my video said “not enough air”… is there anything else I should be looking at that controls air getting into the engine and would malfunction when hot?

 

I might try capping the vacuum lines from vapor canister, PCV, fuel pressure regulator, and make sure the brake vacuum line is solid. Use scanner to monitor short term fuel trims. Check TPS for wear right near closed throttle.

 

Today, based on a mix of comments from here and elsewhere I went looking for other sources for the issue. I pulled the IACV and cleaned it with brake cleaner, removed a little bit of carbon, but there was not much to see of the sensor itself, reinstalled it with no issue.
I also tested the fuel pressure at the rail again both with and without the vacuum line to the pressure regulator valve disconnected and plugged.

At idle with the regulator working: ~30psi steady
With throttle applied: ~25psi steady (it drops)
At idle with regulator vacuum plugged: ~45psi steady
With throttle applied ~45psi steady (no change)

Is it possible the fuel pressure valve is bad? If it goes bad will it starve the engine of fuel by releasing too much pressure?

I also checked all around the engine for vacuum leaks, this time with propane, didn’t get any results, so I don’t suspect a vacuum leak.