• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

LH2.4 guru's - Random stumble at idle

gsellstr

Vintage anti-ricer
300+ Club
Joined
Oct 21, 2004
Location
BFE Desert east of Cali
Had a very slight misfire forever on my wagon (90 745T), but it has changed to a full blown stumble at idle now, almost stalling the motor sometimes, depending on load.

Conditions:
Engine warm, running closed loop. Worse after a 5-10 minutes of driving than just sitting in the driveway idling. Doesn't matter what gear the trans is in. Sometimes stumbles when the e-fan kicks OFF, not very often.
Idle is smooth until the ECU goes closed loop, then the misfire starts.

Backstory:
Been fighting knock enrichment issues for 8 years. About a month ago while driving to work I got a CEL for knock sensor. Cleared and it reset each time, first time it saw 2000rpm. This is on a chipped EZK with desensitized knock circuit. Installed a stock non-EGR EZK, no issues at all. Also NO issues with knock enrichment, so full boost, AFR's mid-upper 10's, pulls strong and clean. Hasn't done this in years. No other work done to the car for a few months prior.

Checked:
Grounds checked at ECU/EZK boxes, cleaned at intake.
New NGK plugs, old ones were slightly black from running rich in boost but looked good otherwise.
Wires ohm out, have around 30k on them, new Bosch cap and rotor.
Bypassed MSD6A, no change.
Power supply from RSR to injectors shows 6.3ohms including the ballast, on all 4 injectors. Injectors show 2.2ohms.
3 different AMM's, no change.
2 different O2's, no change.
ECT checked out at 3700ohms at 50*, 331ohms at 185* operating temp, checked at EZK and ECU.
2 different RPM sensors, no change.
Noticed the lights flicker at idle, despite steady 14.4v. Checked AC at the battery, .06-.22vac, need to check at the alternator. Fluke spec's .5vac max when checked at the alternator. Alternator is a 170A Mechman.
Haven't checked compression in a year, last check was 175 across the board.
Haven't checked valve clearances for a year, last check was .016-.018" warm. Vacuum is smooth and stable at around 18" in neutral with a warm engine.

Current EZK is a TLAO chipped 147 gold box, ECU is TLAO chipped 967.
AMM is 012", running stock airbox.
Injectors are #65 Delphi running 4-bar rail pressure with a Walbro 255 in-tank, straight 91* pump gas from a station I've been using for years, no issues on the wife's car.

I would consider fuel supply except the issue starts when the ECU goes CL. Same for secondary ignition.

Plan (when the rain stops) is to check ripple voltage at alternator, see if that's an issue given the lights flickering. Also need to check fuel pressure. Probably try swapping power stages as well as RSR.

Open to any other thoughts.
 
A local had a similar problem with his 1991 740. Similar setup. Big MAF and big injectors. I think the ECU is probably just getting confused. Same thing, ran fine in open loop, but once it warmed up/went into closed, idle issues. He never figured it out.

All I can suggest is to put some factory stuff back in there and see what happens.
 
Not feasible to go back to factory stuff at this point in time. Only thing that's really different from stock right now is amm and injectors, plus a set of chips.
 
Try stock injectors and MAF. I have read more and more stories of poor running issues with larger MAF. It is one reason I never installed one. It will alter the load signal and ignition timing may not match fueling. Or maybe I am just talking out ass. Always an option!
 
Sorry Zach, adapting the stock amm is a pain on my current setup. Also don't have one or plumbing, which means spending north of $75 just to test. Fwiw it still had the miss when I bought it, bone stock sans the vx.

I am well aware of what the amm swap does, altered timing, adjusted fueling, shifted tables, etc. None of that explains a random miss only at idle and only cl.
 
Once I had somewhat "sticky" injectors, maybe due to corrosion or something that got dissolved by the E85. Problem represented itself in stumbling idle when warm, very rough and erratic acceleration off idle but once the engine had about 2-3000rpm it accelerated mostly normal until it began cutting out at higher revs and load. The injectors coils had spec resistance but in the test stand the fuel distribution was very uneven, intermittend and depending on duty cycle and fuel pressure :???:. Those where genuine bosch injectors with 470ccm/min flow (if I remember correctly).
 
Very well done post.

Sometimes stumbles when the e-fan kicks OFF, not very often.

Ah hah!

Noticed the lights flicker at idle, despite steady 14.4v.

Just got through listening to a story about a Chrysler making the rounds of shops and dealerships before a scope found the alternator to be "confusing" the ECU. 2nd hand story so I couldn't ask my usual impertinent questions about root cause.

Plan (when the rain stops) is to check ripple voltage at alternator, see if that's an issue given the lights flickering.

This would be my plan too.:nod:
 
Plan (when the rain stops)

I hear ya!

hattrick339.jpg


Thankful it ain't snow.
 
I had a severe stumble (more of a real big miss) when my a/c pusher fan clicked out. I ended up putting some big reverse EMF diodes around it and its relay, plus some ceramic caps across those. That solved it entirely. Mine was pretty bad, like it was causing the ezk to lose sync for an entire rev. This was caused when I rewired some stuff and sourced current from the back of the alternator for the ac compressor and fan, but it was a bit of a pain to pin down. Not suggesting it'll help you, but maybe there's a lightbulb in there somewhere.
 
Aware of the spec Zach but thanks.

Brad... Sounds exactly like what mine does. I suspected some form of spike confusing things when the fan kicks off. Digging I to the grounding on the fan, power already comes from the battery so that should be clean. At some point I may look into a soft start setup for the fan.

Injectors sticking sounds plausible but this issue has been there on 3 different sets of injectors, between cleaned greens, cleaned Browns, and new Delphi's. Also wouldn't be sensitive to closed loop only... Theoretically...

Rain is gone, sun is out, digging into it after some grub.
 
Okay folks, more testing, no resolution but a few test results.

Ripple voltage from the alternator is .1vac, so nothing to worry about.
Swapped power stage, no change.
Swapped a 3rd and 4th amm, different running condition but the stumble remains.
Unhooked knock sensor, no change.
Unhooked IAC, idle jumped. Capped the IAC off, found base idle was low, adjusted and adjusted TPS to match. Idles better without IAC than with, oddly. May consider running it without the IAC, need more time to confirm. After idle adjustment it doesn't want to completely die like it did, still stumbles pretty good when the fan kicks on.
Voltage to and from the AMM is in spec, within .1vdc of battery voltage to the resistor pack and to the AMM.
Pin 3 at the AMM was at 2.60-2.65v at idle.
Fuel pressure checks out, rock solid without vacuum, fluctuates slightly with engine speed fluctuations.
Pressure tested the intake tract from the turbo back, no leaks.
I did notice the timing randomly jumps around 5* advanced, seemed like it jumped prior to each stumble, really hard to discern if it's an effect of the stumble (EZK trying to advance timing to increase idle) or the cause of the stumble, over-advanced timing causing it to misfire. While the IAC was unhooked, engine speed was around 1000rpm, timing was rock solid.

And now with video...

<iframe width="420" height="315" src="https://www.youtube.com/embed/2JvuOOzN4pg" frameborder="0" allowfullscreen></iframe>
 
Did you figure this out?

I had a similar problem, that eventually progressed to the car not running in closed loop. Once the problem started, I did a lot of troubleshooting - I changed various sensors, replaced the AMM with known good ones, checked voltages, etc. and everything checked out fine. In desperation, I rewired the harness extension to my AMM*. I had a spare harness around, made sure that I got 4 feet of wire attached to the connector, cut, soldered, heat shrunk everything properly. That fixed it.

Next time I do one, I've got some coax cable to use for wiring and I'll tie all the shielding to ground. I've got a hunch that with all the extra wire to the AMM, the circuit becomes much more vulnerable to noise (those wires are like 4 foot antennas), and some random corrosion or whatever is the straw that breaks the camels back. A multi-meter wouldn't pick that up and I didn't hook it up to a scope, so it's just a hunch.

*Previously, I was using an extension that plugged into the existing AMM connector by the intake. It had a scavenged male connector from a destroyed AMM on one end, and the female on the other. The wire between was 14 gauge multi-strand. There were no shorts that could be detected with a multi-meter.
 
Honestly, the initial cause for the thread has been resolved, still have an occasional stumble but not nearly as bad. Been meaning to update the thread.

The cause of the stumble/die ended up being battery. In the process I ended up running #4 from the battery to alternator, #4 to the starter, instead of the previous #8 to the starter, then #10 from the starter to the alternator. That alone helped things along, plus some other 'aged' connections to the harness and such were cut, new ends crimped or soldered, then heatshrinked, so those issues are taken from the equation. After doing that and readjusting the throttle plate, swapping in a new idle motor, the idle improved and much of the misfire disappeared as well, but it would still stumble and die when the fan kicked on or any decent load.

Ended up buying a clamp-on ammeter (been on my wish list for years anyway), confirmed the fan had an initial spike around 35-40A, constant of around 17A. Compared to the spare, very similar, compared to the wife's R, very similar. Cool, fan is fine. Charge voltage was around 14.2v, ripple was around 0.1VAC, rest voltage was 12.25-12.3v. Bingo. Finally replaced the Red Top with a date stamp of april 2005 with a new Super Start Platinum. The optima when tested (had to force the tester to run it due to low rest voltage), came back at 550CCA after a 5-second test so yeah, it's time had come.

As of now I can hit the system with around an 80A instantaneous draw, idle will drop around 150rpm if in gear while the alternator spools up, then idle comes right back up. Tested the system with every electronic device turned on, 120A at idle, with the Mechman alternator charging along happily at 14.1v at idle with an underdrive pulley on the crank. Damn fine unit!
 
Idle is smooth until the ECU goes closed loop, then the misfire starts.

On GM vehicles

BACK-UP MODE (also known as limp-home mode)

The back-up mode is used any time the ECM cannot operate normally. In this mode, the ECM looks to the CALPAC chip to determine engine operation. The CALPAC is a chip that contains minimal information the ECM can use to allow the engine to run using only distributor reference pulses, throttle position, and coolant temperature inputs to change fuel and ignition timing calculations. Back-up mode was designed to allow the vehicle to “limp-home” and not leave the customer stranded should a major problem occur. The ECM will implement back-up mode if any one or a combination of the following conditions exist:

ECM input voltage is below 9 volts.
Cranking voltage is below 9 volts.
PROM missing or not functioning correctly.
ECM internal circuitry fails to ensure proper Computer Operating Pulses.
===================

So, maybe voltage fluctuations were causing a change from closed loop mode to limp home mode...back and forth
 
Back
Top