Miss firing Merc 7.4 mpi L29 2000 2855 Ciera SB

[Magonsterz]

Engine serial number 0L373392
Merc 7.4 mpi 2000 2855 Ciera SB

Started missing, out of the blue, had been a while since tune up so replaced rotor and cap (showed corrosion) still has miss.
Pulled plugs and port side plugs looked to be covered in black soot but dry starboard side plugs were correct cocoa brown color. Wire brushed, hit with brake clean, gapped to .045, reinstalled. Plan to replace with new.
Replaced plug wires with new, even swapped in new fuel/water separator filter, ran better but still missed, just less.
Had new ignition module and dist coil pick up and ignition coil from previous project build, swapped in and set timing to 8degress, starts right up, comes up to temp 170 on gauge cluster, idles and revs just fine but still has intermittent miss regardless of at idle or up in the rpm, still sooting up all port side 1, 3, 5 & 7 plugs only.

Figured best to check for codes at this point, came up with code 14, "Engine Coolant Temperature (ECT)" so going to run the traps on Sensor and Sensor Circuit - Low Temperature Indicator testing procedure unless any one has different advice for me.

Using Merc service manual Number 23 pages 783-785
https://d3gqux9sl0z33u.cloudfront.ne..._8.2_litre.pdf

 

[nola mike]

Looks like a pretty good flow chart. Bad ECT would cause it to run rich, not sure why it would only affect one side though. Does it miss only when warm?

 

[Magonsterz]

Looks like a pretty good flow chart. Bad ECT would cause it to run rich, not sure why it would only affect one side though. Does it miss only when warm?

at first would only when warmed up and drop out of closed loop, the started missing regardless of start up or warn, or rpm band.
going to swap in new plugs as had originals in it, 300 plus hrs so why not, cheap insurance, hopefully end of issue, if not picked up ect just in case have to continue expedition, one less run to parts store in the middle of wrench time lol, to be continued, thx for he input.

 

[Magonsterz]

update, replaced with new plugs, mild improvement but still surges up and down and misses, starts, comes up to temp, once comes off closed loop, pulled codes, none only 12, so swapped in new ECT for the hell of it at this point, no change. Once warm ran it up to 2500 rpm, no problems spooling up and holding then all of sudden surge up then down and what seems like miss fire, throttle down and same at idle just less prevalent.
Is it possible knock sensors, fuel pump or injectors may be the issue and would any of those item NOT trigger a code since only one registering is 12?
stumped at this point.

 

[Magonsterz]

update, replaced with new plugs, mild improvement but still surges up and down and misses, starts, comes up to temp, once comes off closed loop, pulled codes, none only 12, so swapped in new ECT for the hell of it at this point, no change. Once warm ran it up to 2500 rpm, no problems spooling up and holding then all of sudden surge up then down and what seems like miss fire, throttle down and same at idle just less prevalent.
Is it possible knock sensors, fuel pump or injectors may be the issue and would any of those item NOT trigger a code since only one registering is 12?
stumped at this point.

Current status
After complete, new distributor, new plugs, new wires, new ECT sensor, throttle body clean out, vacuum test fuel pressure regulator and good vacuum signal while running (18 lbs. ) still has intermittent stumble cold or up to temp.
Only thing i have left to do is pressure test fuel pressure both initial key on and running, record any changes as stumble occurs.
Is there a way to test IAC? If malfunctioning wouldn't it record a code?

If fuel pump and IAC checks out thinking Injectors?

 

[nola mike]

You had a code, doesn't seem like you did the troubleshooting for it.

 

[Magonsterz]

only code recorder were 14 and 12, 12 only indicates pcm is in diagnotic mode and communicating correctly, 14 is ect issue. Tested not found at fault but swapped out anyway. Erased codes, started, brought up to temps and through the rpms, still has miss regardless of rpm band, cold or warmed up.

Did notice and odd whine prior to std fuel pump whirring when system key on initially primes fuel system, thinking it may be intermittent fuel pump starting to fail, but it will not cause any fault code to indicate pressure spike or loss, waiting on pressure gauge to attach to fuel rail and see if gives pressure variance indication while running and misfiring.

 

[nola mike]

only code recorder were 14 and 12, 12 only indicates pcm is in diagnotic mode and communicating correctly, 14 is ect issue. Tested not found at fault but swapped out anyway. Erased codes, started, brought up to temps and through the rpms, still has miss regardless of rpm band, cold or warmed up.

So you tested the wiring as well?

 

[Magonsterz]

So you tested the wiring as well?

yup, full signal contact from pcm to ect as when comes up on temp it drops out of closed loop yet will miss irrelevant if in closed or open loop.

 

[nola mike]

yup, full signal contact from pcm to ect as when comes up on temp it drops out of closed loop yet will miss irrelevant if in closed or open loop.

You've mentioned closed/open loop a few times...afaik your engine doesn't use a closed loop system... But are you no longer getting a code 14?

 

[Magonsterz]

You've mentioned closed/open loop a few times...afaik your engine doesn't use a closed loop system... But are you no longer getting a code 14?

correct, only 12.
waiting on pressure gauge to show up to test fuel pump in conjunction with my vac gun to test against pressure reg and finally monitor system pressure while actually running and actually stumbling.

 

[Magonsterz]

correct, only 12.
waiting on pressure gauge to show up to test fuel pump in conjunction with my vac gun to test against pressure reg and finally monitor system pressure while actually running and actually stumbling.

hoping it isnt a boogered up injector......that we shall see

 

[nola mike]

Only thing i have left to do is pressure test fuel pressure both initial key on and running, record any changes as stumble occurs.
Is there a way to test IAC? If malfunctioning wouldn't it record a code?

If fuel pump and IAC checks out thinking Injectors?

Admittedly ignorant about the merc systems, so just throwing out my ill informed thoughts...
IAC shouldn't have huge effect once your rpms are up. You can check the operation with a scan tool, number of counts. An open loop system kind of limits your ability to tell whether you're running rich/lean, and I don't think Merc is sophisticated enough to tell you if you're having cylinder misfires. Definitely need to check fuel pressure and put a timing light on each plug wire to make sure you're getting a consistent spark

 

[Magonsterz]

Admittedly ignorant about the merc systems, so just throwing out my ill informed thoughts...
IAC shouldn't have huge effect once your rpms are up. You can check the operation with a scan tool, number of counts. An open loop system kind of limits your ability to tell whether you're running rich/lean, and I don't think Merc is sophisticated enough to tell you if you're having cylinder misfires. Definitely need to check fuel pressure and put a timing light on each plug wire to make sure you're getting a consistent spark

have an inline plug test light on order to visual the fire/misfire episodes.

 

[dubs283]

Your system does not operate on open or closed loop. That is for emissions controlled (catalyst) engines. FYI a catalyst system starts and warms up in open loop then changes to closed loop for normal operation. Saying "comes off closed loop" makes no sense, this would only happen if there were an issue with the efi system and the pcm defaulted to open loop based on such. Also, mercs MEFI systems were pretty simple and didn't offer much in the way of diagnostics. Basically if fhere are no active faults, the system is good.

Your symptoms sound very much like fuel pressure issues. As you stated you need to check the fuel pressure at static, idle and speed under load. Possible there is a pump/regulator issue.

Fuel injectors rarely have an intermittent problem. If they're clogged there clogged, similar with operation, basically they work or they don't. Cannot rule out a wiring issue though, a wiggle test can help with that

 

[nola mike]

Your system does not operate on open or closed loop. That is for emissions controlled (catalyst) engines. FYI a catalyst system starts and warms up in open loop then changes to closed loop for normal operation. Saying "comes off closed loop" makes no sense,

I'd probably call the merc an open loop system, since the pcm is operating on a fixed maps based on some sensor input. It just doesn't ever go into closed loop. But yeah, he isn't ever switching into closed loop

 

[Magonsterz]

I'd probably call the merc an open loop system, since the pcm is operating on a fixed maps based on some sensor input. It just doesn't ever go into closed loop. But yeah, he isn't ever switching into closed loop

gotcha, i was thinking it was in closed until it warmed up and switched to opern once ect saw temps above 160 and then went open i see that it now isnt the case. thx for clarity. NOOB

 

[Magonsterz]

finally all back together with fresh fuel, firing order and timing are spot on, fires right up.
fuel pump replacement fixed low pressure problem now rock solid 38 at idle, jumps up to 43 when throttled up.
now onto next problem, all four port side injectors confirmed not firing as was noted before but unable to confirm until fuel pump pres. issue was resolved.
I can fire it up and unplug each wire one by one and there is zero change in engine on all port plugs and typical stumble on all starboard sides.
spark present and confirmed to all plugs with inline probe and a few good zaps!
no codes are present and not inclined to believe all four injectors (1, 3, 5, 7) on same bank of engine failed at the same time, have yet to test signal or lack there of at injector plug.
injector fuse is good as there is only one and all starboard side is firing correctly.
leaning toward ECU Injector driver.
Any suggestions on how to best test my theory would be greatly appreciated.
Is there a place i can send ecu to for diagnosis or refurb? 2000 7.4 mpi, MODEL: DELPHI 16237009, engine sn 0L373392

 

[achris]

leaning toward ECU Injector driver.

Batch fired injectors. I'd be looking for a broken wire (or solder joint). Don't forget to check the negative side too.

Hang tight and I'll find a diagram that might help.

Chris.......

 

[achris]

Here's a snip of the diagram. Check that connector, and the joints (both positive side and negative side)


This is a simplified 'block diagram' of the system.