Engine running rich and random misfires
 

This is a problem that I have been trying to diagnose for the better part of 6 months now. 2000 SL, stock everything. No codes stored in ECU. Here are the symptoms:

• Relatively poor fuel economy
• Random cylinder misfires
• Random popping in exhaust
• Very negative fuel trim
• Rough Idle

I replaced the ECTS a while back with a new brass one, and that seemed to help with the fuel economy slightly. I have changed the spark plugs, checked the spark plug wires for spec'd resistance and continuity, measured resistance across coil packs, cleaned fuel injectors, cleaned IAC, TB, and EGR, replaced air filter, changed fuel filter, Repaired small crack in exhaust just before flex tube, checked FRP for fuel leaks after removing vacuum line, checked Fuel injectors for leaks (1 does leak slightly but still responds to voltage pulses properly-- it only leaks when the rail is pressurized and it is not receiving voltage. When I say small leak I mean like 1 drop every 60 seconds).

Everything seems pretty normal. I have the Torque app for android so I can see when misfires occur. They seem to go pretty much hand in hand with the popping in the exhaust. The weird thing is, they don't occur when the engine is under load. Even with just the AC compressor on, it smooths out the idle and no misfires occur. Same thing when driving.

While idling, I generally see fuel trims between 0 and -5, which seems pretty good. While driving they are anywhere from 0 when I let off the gas, to -30 when accelerating lightly. I'm assuming that is where the bad MPG is coming from.

Anyone have any ideas? I feel like I have covered about everything...

Your idle around 950 or higher. Let it idle for two minutes and you will get a check engine light for a P0507, idle more that 200 rpm high for 2 minutes. You need a new intake manifold gasket.

Confirm this by spraying something like wd40 or a carburetor cleaner around the gasket around the #1 cylinder to see if the idle is affected. Any change means the gasket is leaking. Replace with a felpro gasket, not a factory gasket. There is no support for the gasket around the #1 cylinder at the top outboard corner so the factory gasket is too pliable for that area. Felpro is metal backed and stiffer.

Sprayed brake cleaner all around the intake manifold and saw no change in RPM. I'm wondering if it could have something to do with the EGR valve? Just a guess.

I have already replaced the ECTS. I have verified that it is giving accurate temperature readings as well.

When fully warmed up, what does the engine idle at in neutral? When you spray something, you spray it on the gasket where the manifold and the head meet. The problem area is very difficult to get to as it is covered by the power steering pump.

My Saturn does not have power steering, so the area is easier to get to. I sprayed around the area where the intake manifold bolts to the head. When at normal operating temp (around 160 degrees with cooling fan off, 130 w/ cooling fan on) it idles between 820-850 RPM. The slower the idle speed gets, the more noticeable the missing gets.

Can you perform a compression test? If so, get the numbers, and post them here ...

160 degrees sounds a little cool for the engine to me. I'm pretty sure that they originally had 180 degree temp thermostats in the 1.9. You might try replacing the thermostat with a new 180 degree one.

I get within 2 or 3 psi of 120PSI on every cylinder.

Looking over data from my OBD software, this is what I am seeing:

-Intake manifold pressure remains between 4 and 5 PSI
-Commanded AFR Remains around 14.7:1
-EGR Duty cycle % remains at 0, EGR voltage steady at .7v
-Coolant temp w/o cooling fan hovers around 230 before kicking on the fan
-ST Fuel trim fluctuates between -15 and -30%
-Timing advance constantly fluctuates between 10 and 21 degrees
-Vacuum gauge remains steady around 10 PSI

Anything else that might help?

In case theres a chance it will help with troubleshooting, I have included a Mode $06 report.

Test report:
------------------
TID:$02 CID:$06
- Lean to Rich sensor threshold voltage(constant)
Min: 33,118
Test result value: 32,768
FAIL
----
TID:$02 CID:$50
- Lean to Rich sensor threshold voltage(constant)
Min: 32,693
Test result value: 32,693
PASS
----
TID:$02 CID:$52
- Lean to Rich sensor threshold voltage(constant)
Max: 32,768
Test result value: 32,768
PASS
----
TID:$02 CID:$60
- Lean to Rich sensor threshold voltage(constant)
Max: 38,768
Test result value: 32,768
PASS
----
TID:$02 CID:$66
- Lean to Rich sensor threshold voltage(constant)
Max: 32,888
Test result value: 32,693
PASS
----
TID:$02 CID:$71
- Lean to Rich sensor threshold voltage(constant)
Max: 32,693
Test result value: 32,693
PASS
----
TID:$07 CID:$4d
- Minimum sensor Voltage for test cycle(calculated)
Max: 32,768
Test result value: 32,768
PASS
----
TID:$0a CID:$01
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$03
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$04
- Sensor period(calculated)
Min: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$05
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$06
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$07
- Sensor period(calculated)
Min: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$42
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0a CID:$48
- Sensor period(calculated)
Max: 43,690
Test result value: 43,690
PASS
----
TID:$0c CID:$60
- Misfire counts for last/current driving cycles(calculated)
Max: 33,268
Test result value: 32,768
PASS
----


End of report.