Rough idle/part load. HELPPPP

milestonemotors

New Member
Joined
Sep 25, 2020
Okay gentleman, I need the help of the pros. I have an 86 grand national, its running rich/rough at idle & part load. While at idle the AFR is low 10, while at part load it'll drop to 8 & 9 and begin to break up. If I hold WOT it'll clear out at be right at 12 and change. The specs are the following: It has an AEM 340 fuel pump, acuufab FPR set at 43PSI, street chip from TT. OEM 3in LS1 air mass meter with the translator. A/c delco CR43ts plugs.
Scanmaster readings while at idle warmed up:
injector PW: 7.3-7.5
No trouble codes
MAF: 9.4 to 9.7
LV8: 65-70
Volts: 13.6
INT: 128
BLM: 142
ALC content: 34
Coolant 166.5
Air temp: 79.4
RPM: 775
TPS: .42
IAC: 58
02: 144
Spark advance: 26.5

I smoke tested the intake- No leaks. All 6 plugs look equally in color, pretty dark. Ive replaced them as well, no change. I tested the spark at the coil and the end of each wire with a noid light. All 6 are operating. Fuel pressure is holding 43 at idle. The scan master readings are kind of gibberish to me, If someone could point me towards what to test it would be much appreciated!
 

Chuck Leeper

Toxic old bastard
Staff member
Joined
May 28, 2001
" bad O2 sensor, are common causes".
TT chip go open loop @ WOT?
If so, that could go to the O2 sensor at <WOT, messing with the AFR.
 

milestonemotors

New Member
Joined
Sep 25, 2020
BLM is to high...you have a pre turbo exhaust leak(s)
IAC is to high...should be 10-20
Hi Scooby, thank you for the info. I did notice some black soot around a few of the spark plug wires Indicating an exhaust leak. Do you think the high IAC is because of the exhaust leak as well? If so could you explain for me to have a better understanding of how it’s compensating? Much appreciated!
 

milestonemotors

New Member
Joined
Sep 25, 2020
" bad O2 sensor, are common causes".
TT chip go open loop @ WOT?
If so, that could go to the O2 sensor at <WOT, messing with the AFR.
Hmm I’m not sure if it goes into open loop at wot. If so & the o2 sensor is defective that would make sense Why it’s clearing out. I’ll look into it. Thanks boss
 

SCOOBY DOO

I'M NOT A MONSTER, I'M JUST AHEAD OF THE CURVE!!!
Joined
Feb 18, 2007
There's a bunch of physics involved in it but pre-turbo exhaust leaks actually suck air in, head faking the ECM into thinking the engine is running lean, so it richens the AF to compensate, throwing the BLM's for a shit.
 

milestonemotors

New Member
Joined
Sep 25, 2020
Exh leak prior to the O2 sensor, will cause a sensor to indicate lean, as the exh stream has unburned oxygen in it.
Therefore, the system adds fuel.
Ahh This all makes sense. I also noticed someone has done some sort of welding repair on the headers before, I’m going to replace them altogether. Are there any headers that you recommend?
 

milestonemotors

New Member
Joined
Sep 25, 2020
There's a bunch of physics involved in it but pre-turbo exhaust leaks actually suck air in, head faking the ECM into thinking the engine is running lean, so it richens the AF to compensate, throwing the BLM's for a shit.
thanks boss, I’m going to tackle the headers this week. Any recommendations for headers? Mine already have welds on them from some sort of repair from the previous owner
 

SCOOBY DOO

I'M NOT A MONSTER, I'M JUST AHEAD OF THE CURVE!!!
Joined
Feb 18, 2007
thanks boss, I’m going to tackle the headers this week. Any recommendations for headers? Mine already have welds on them from some sort of repair from the previous owner
They're mild stainless steel. Find a welder that knows his shit. Don't hand them to some back yard welder. You could have leaks at the 2 flanges and the base of the turbo. Several of the vendors sell turbo flange gaskets.
 

Chuck Leeper

Toxic old bastard
Staff member
Joined
May 28, 2001
If they've been "welded", there's a good chance they have cracked right at the edge of the weld.
A smoke test should show the leaks.
In addition to the dr side cracks, the bellows on the pass side crack in the grooves.
In addition to the above mentioned leak sources, add in the cross over flange connections.
My $.02.....Spend the bucks, and get a new set.
 

Pronto

You can't knock them up with spit.
Joined
Dec 9, 2002
Probably worth the effort to remove the valve covers and loosen the rocker shaft to shut all the valves. Smoke the intake from the turbo inlet and smoke the tailpipe up to the head.
This is simple enough:
 

SCOOBY DOO

I'M NOT A MONSTER, I'M JUST AHEAD OF THE CURVE!!!
Joined
Feb 18, 2007
It'll leak at the diaphragm or mating surface or both. Erics chip shuts off the EGR so you can use a plate sold by several Buick vendors.
 
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