Outatimemk

1999 C2 with ~101k miles, 6MT.

Hi all,

I'm in need of some help and I'm going to put as much information as I can in here in the hopes that the combination of all of this makes sense to someone and can help me figure out what's wrong. I've been chasing down an issue over the better part of 6 months and I'm really trying to lean hard into analysis of it so that I don't continue throwing parts at the problem. That being said, I have replaced a LOT of things to date. I've owned the car since 2020, and the biggest job I've done is an engine-out to do the IMS, RMS, Variocam pads, any gaskets/seals I could think of, and many other things. Without further ado, here it is.

The Problem
On a cold start, while warming the engine for about 10-20 seconds, the idle will drop from its ~1200 cold start idle, and sputter down to ~500 and either come back up or stall, independent of me doing anything like hitting the gas. Restarting the car usually works and the car will run without issue. This part of the issue is intermittent and random and I cannot say for sure whether it happens more frequently when I am below a half tank of gas (more on why that matters a little further down). It also smells awful like it's running overly rich.

On a hot engine, independent of ambient outside temp, I will clutch-in while coming to a stop and I see the idle dip below the regular idle speed of ~680RPM and either sometimes come back up and fluctuate, or the car will stall. This has happened with either the AC on or off. Most recently, I have noticed that this tends to happen when the gas tank is at half full or less. Even though that's been the most recent development, I BELIEVE the car has stalled at any other point in the fuel gauge, but I've replaced many parts so it's possible those were also problematic and resolved things to a degree, as the car has ran much better. The latest stall happened at 1/4 tank while I was on the way to the gas station. I filled up the gas, and I have yet to see a stall again. Upon filling up the car, I noticed it ran MUCH smoother. I am currently at half a tank. I can keep running it down and see what happens.

CEL Codes
P1123 Oxygen Sensing Range 1 Cylinder (1-3)
P1125 Oxygen Sensing Range 1 Cylinder (4-6)

Replaced Parts/Diagnosis
Crank Position Sensor (2023)
MAF (2023)
Fuel Filter (2023)
All 4 O2 Sensors (2023)
AC Recharged After Engine out (2023)

Intake was smoke tested in November and found minor leaks - fixed; and again 2/9/2024 - no leaks this time

ICV Cleaned with carb cleaner and tested with 9V and ground to pins to see the doors moved freely. TB was cleaned as well.

Engine Out from 2022-2023:
UAOS
IMS Solution/RMS
Thermostat/Waterpump
Variocam pads Banks 1 & 2
All Fuel Injectors (that seem to be discontinued now): https://lnengineering.com/new-afterm...280150455.html
Upper & Lower chain pads
Cam chains
Vacuum lines with Y fittings
AOS Connection Piece on Bank 2
Plugs/Coil Packs

Durametric Numbers
Read out was done on a nearly hot engine:
Oil Temp 76* C
Engine RPM: 680, Stable
Range 2 Cyl 1-3 FRA: 0.90
Range 2 Cyl 4-6 FRA2: 0.92
Range 1 Cyl 1-3 TRA: 0.0 (I have also seen -0.34/-0.37 TRA/TRA2 on another run and I think that was a colder temp at 46* C)
Range 1 Cyl 4-6 TRA2: 0.0
Camshaft position 1 Deviation: 0.0
Camshaft position 2 Deviation: 5.0 (why wouldn't this be 0.0? It's within spec, but did I not time Bank 2 right after replacing the variocam pads?)
Oxygen Sensing cylinders 1-3: 0.98 - 1.02 (fluctuates)
Oxygen Sensing cylinders 4-6: 0.98 - 1.05 (fluctuates)

Fuel Pressures
Here's where it may get interesting:

Prior to start: 48PSI (stable)
Running: 45PSI (stable)
Revving to 2000: ~53PSI, drops to 40PSI and stabilizes at 45PSI
Engine off: Drops from 45PSI to 35PSI and then climbs to 48PSI within a minute
Engine off for 1 hour: 44PSI
Engine off for 3 hours: < 10PSI

If I'm to understand correctly from whatever is out there on the internet, a 1999 C2 should have idle fuel pressure at 55PSI? If someone can confirm what it is really supposed to be, that would help.

At this point, I am leaning toward something in the fuel system but something still doesn't seem right. The fuel pressure does not really fluctuate at idle so that potentially means the FPR is fine. The pressure is generally low (need to confirm that) so that could mean fuel pump; but, a fuel pump usually just dies and doesn't have low-pressure pumping issues. It certainly smells very rich at startup so I am not sure why a failing fuel pump would cause a rich condition.

I can provide more screenshots/pictures/info you need to get a clearer idea of what's going on. Any help/thoughts are appreciated!

DBJoe996

When have you cleaned your ICV (idle control valve) and TB? Seems you are leaning towards a fuel pressure problem but it could just be a gummed up ICV.

De Jeeper

The cam.deviations do look suspect but i dont think this is your issue.

The 2 things i would go after first is cleaing the idle air orifice in the tb and do a smoke test in the intake. Your o2 values r going high so the computer is adding fuel probably cause by an ait leak. U can kind of see this in the short term fuel troms but as they should be closer to 1. Although your fuel pressure is also a suspect at the pressure ranges u r posting. Low fuel pressure can wrek havok on the o2 and idle quality.

Check the first 2 because u dont have to buy parts and then maybe replace the fuel pump.

Outatimemk

I forgot to put that on there but yes I did clean the ICV with carb cleaner and also applied 9V and ground to the pins to see that the door inside it moved freely.

I also smoke tested the intake yesterday and I can see no leaks at all. I have seen leaks before and fixed those, but the issue remains. I’ll update the original post.

De Jeeper

I can remember what the maf range is suppose to be but u cam look it up. I would graph it through the rpm range and then back to idle. There was a rash of bad maf sensors last year even though they were all bosch certifided.

Outatimemk

So to do this, wait until the engine is hot, read out the durametric value and hit the gas?

De Jeeper

yes even a couple hundred feet down the street so u "load" the motor. See if the sensor falls out of range.

De Jeeper

U could always remove amd inspect the fuel pump. There have be instances that the internal tubes have split and since your issues r when the fuel level is low its a possibility. Its pretty easy to access

allcool

↑ THIS ↑

But if it were me, @ over 100,000 miles, once i open that fuel pump hat... I'd just replace the pump and the lines as needed. Old lines do split, especially with our current modern high alky fuels.

You've got the easier to do 2wd simpler fuel pump/lines system.

jumpy chunky

Fuel pressures on your 99 seem to be okay.
The static pressure will be 55 psi., running pressure around 45 to 47.

Seems a little counterintuitive to think there’s a fueling malfunction when trouble codes indicate over fueling based off of O2 sensors. OP states there’s a fuel smell on cold start. Also car might stall when fully warmed up.
Sounds as if cats are compromised in some way.
Try to log voltage of post cat sensors. Then post findings if you can. Also MAF readings should be 15 to 17 KG per hour at idle and voltage should be around 1.4

Porschetech3

The TRA reading here is the KEY....The reading of 0.0 is not plausible and was probably due to clearing the fault codes, that re-sets them to 0.0 ...

The previous readings of -0.34 and -0.37 means the TRA went full lean trying to compensate for a RICH condition....

This is happening on both banks, ( P1123 P1125 )the two most common things I have seen to cause this is MAF Sensor and Fuel Pressure regulator "leaking fuel up through the vacuum hose"

Pull the vacuum hose off the fuel pressure regulator while running and see if it has fuel in it....( not while engine is extremely hot for safety sake)

I know you replaced the MAF, but what part number did you put on and was it a Genuine Bosch MAF?

But rest assured the engine is running RICH and most likely doesn't have a vacuum leak...

Edit :::FWIW Codes P1123 and P1125 can be either Rich or Lean , it depends on if the TRA went negative (-) or positive...in the PIWIS, the fault code is accompanied by a subtitle that says either "above" or "below" and can get real confusing, but just looking at the TRA for a negative ( -0.36) or a positive (0.36) will keep the confusion down..

Porschetech3

TMI ?

mccahey

You can never have TMI !

Outatimemk

No sir! Testing right now but running into issues with Durametric and my laptop. Plus it’s hard to figure out how to take readings while driving 🤣

Outatimemk

One thing I am thinking of doing is running it down to 1/4 tank again and seeing what that does to my fuel pressure.