4.0 rough idle lack of power, looking for ideas/opinions

Hey guys,

been in this game a long time, I've had both a 1999 and a 1996 ranger before but have been out a minute (long enough to get my old acct shut down). recently i picked up my 1993 4.0 ext cab manual trans for 700 bucks with a bad motor. Installed a junkyard 4.0 along with a host of new parts and it has been a killer daily driver except for an intermittent rough idle and lack of power.

Symptoms: rough shaky "missing idle" lack of power under 2800 rpm, poor fuel mileage. sometimes it is smooth as butter and runs with great power, sometimes sitting in the drive through and idling it catches itself and will idle up and smooth out, sometimes it wont.

Codes: After fixing the bad battery, the only persistent KOER codes are HEGO o2 always rich. no other codes are present and it will OCCASIONALLY kick a CEL once its warm in the same spot on my morning commute and it will go away in 30 sec or so. no stored KOEO codes

What i have replaced/fixed/inspected:
New Intake air temp sensor
new steel intake tube to replace cracked oem
new motorcraft engine coolant sensor
Fuel pressure key on at 32 psi, will climb with engine running and throttle application to high 30's
Checked intake and plenum gaskets for leaks with carb cleaner
checked engine vacuum for evidence of blocked exhaust (vac good)
run with new MAF and OEM MAF (no change)
tps passes "goose test" when running KOER diagnostic
new NTK o2 sensors
fixed PO's massive exhaust leaks
FPR vac line does NOT have fuel inside of it


The problem has improved, but i mostly credit that to fixing the busted ass intake tube, now im hunting the source of richness and i'm running out of ideas and i don't really have a throw parts at it budget

and the first person to tell me to search is getting pistolwhipped, I've been researching this problem for a few weeks and i'm exhausting a lot of the usual suspects

Pistol-whipped, interesting.. welcome back to TRS

Anyway I don't see catalytic converter on your list.. it's just one of my favorite topics

Denisefwd93 said:

Pistol-whipped, interesting.. welcome back to TRS

Anyway I don't see catalytic converter on your list.. it's just one of my favorite topics

Click to expand...

so i hooked up a vacuum gauge to check running engine vac and it pulls great vacuum the way it should. Seemed to lead me to believe that there was no major exhaust restriction. i temped the cat at the front and back inlets and the temps line up with what a functioning cat should run.

i should also mention it passed an emission sniffer, however that doesn't mean to me that it isn't restricted, just functioning as a converter


edit: i do plan on pulling it for inspection, but im on day 5 of 9 straight at work. I get very little work time on it so i need to organize and have a plan on my first day off

Welcome to TRS

What do the spark plug tips tell you about Rich running?

If MPG is low then fuel is going somewhere, tips will be a darker color if its going into the cylinders.

What codes did you get, 1993 computer should use 3-digit codes.
173 is passenger side rich
177 is drivers side rich

If only one bank is Rich then most likely an injector is sticking and that spark plug should be darker than the others on that bank

Is the rough idle only warm or both warmed up and cold?

Plugged exhaust usually first shows up at higher RPMs/speeds as a lack of top end power, same for lower fuel pressure.

And then your down to the 25 year old computer.................you can pull it and open the top and have a look at the circuit board.
Looks like this: http://www.auto-diagnostics.info/ford_eec_iv

The 3 Blue Capacitors can leak after 25 years, and cause odd issues, under $5 to replace them and clean the circuit board

RonD said:

Welcome to TRS

What do the spark plug tips tell you about Rich running?

If MPG is low then fuel is going somewhere, tips will be a darker color if its going into the cylinders.

What codes did you get, 1993 computer should use 3-digit codes.
173 is passenger side rich
177 is drivers side rich

If only one bank is Rich then most likely an injector is sticking and that spark plug should be darker than the others on that bank

Is the rough idle only warm or both warmed up and cold?

Plugged exhaust usually first shows up at higher RPMs/speeds as a lack of top end power, same for lower fuel pressure.

And then your down to the 25 year old computer.................you can pull it and open the top and have a look at the circuit board.
Looks like this: http://www.auto-diagnostics.info/ford_eec_iv

The 3 Blue Capacitors can leak after 25 years, and cause odd issues, under $5 to replace them and clean the circuit board

Click to expand...

its a 173 so i'll focus on the passenger side equipment, i have a bin of injectors from the last motor so surely one works if ive got a sticky unit

both cold and warm, only kicks the code warm

it actually makes its best power above 2800 once it clears its throat

thanks for the heads up on the computer, it has some strange electrical nuances but thats mostly in the gauges when the hvac blower is on full bore, hasn't seemed to affect performance but if it alters the gauge feed signal then surely its messing with some key sensor feeds to the comp

thanks for the fresh opinions, it's nice to have some direction

Yes pull spark plugs on passenger side and check tips

1993 is full analog system, computer was only used for air:fuel mix and "suggestions" for spark advance to EDIS-6 module the 4.0l used for spark

So dash gauge changes would be electric system issues, could be alternator is leaking AC volts at times or one of the three fields has failed, causing higher and lower voltages, especially with higher electrical demand, and Blower on HIGH is high demand

Dimming head lights at idle means one Field in alternator is off line

Computer uses voltage comparison for sensors, so vehicle voltage can be 12.3 volts(engine off) or 14.9 volts(just after startup) and sensors return voltage don't change when compared, i.e. sensor sends back 1 volt less the system voltage, so 11.3 engine off or 13.9 if system voltage is 14.9, the 1 volt difference is sensor's "data", what computer uses.
And most sensors use 5 volts from a power supply in the computer in any case, so its input voltage doesn't matter up to a point, its always 5v out

Gauges on the other hand use vehicle voltage directly, except fuel gauge which has its own power supply.

RonD said:

Yes pull spark plugs on passenger side and check tips

Click to expand...

definitely something going on in cylinder 2, other plugs were all clean except for 2. Not excessively dark but decidedly different. I'll replace it and go from there.


It looks like the PO was fighting it too, he bought a coil and a set of ebay injectors that didn't fit. Although I don't trust much of his diagnostics, the same person also jb welded the alternator bracket to the head...

FlipFlopFabworx said:

definitely something going on in cylinder 2, other plugs were all clean except for 2. Not excessively dark but decidedly different. I'll replace it and go from there.





It looks like the PO was fighting it too, he bought a coil and a set of ebay injectors that didn't fit. Although I don't trust much of his diagnostics, the same person also jb welded the alternator bracket to the head...

Click to expand...

Yikes that’s hokey as they get lol. I was under the impression these systems should hold 40 psi fuel pressure. The FPRs usually will leak into the vac line but it could be such a leak that it only sends fuel back to the return line and not leaking into the vac line (unlikely however).

Also eBay injectors aren’t ever a good idea for obvious reasons, P.O. obviously didn’t care too much about the truck lol. I’d be inclined to get new injectors or even hit up a JY and get some used OEM ones tbh.


Sent from my iPhone using Tapatalk

cp2295 said:

Also eBay injectors aren’t ever a good idea for obvious reasons, P.O. obviously didn’t care too much about the truck lol. I’d be inclined to get new injectors or even hit up a JY and get some used OEM ones tbh.

Click to expand...

I have had very good luck with eBay injectors. Both my 4.0s are running full sets of Bosch remans bought from eBay.

The truck fouls plugs to the point of needing a can of SeaFoam once a year, but I attribute that to how it gets driven. I basically drive it far enough to open the thermostat and then shut it off right away and let it sit for a few hours twice a week. Doesn't see much use other than that these days.

cp2295 said:

Yikes that’s hokey as they get lol. I was under the impression these systems should hold 40 psi fuel pressure. The FPRs usually will leak into the vac line but it could be such a leak that it only sends fuel back to the return line and not leaking into the vac line (unlikely however).

Also eBay injectors aren’t ever a good idea for obvious reasons, P.O. obviously didn’t care too much about the truck lol. I’d be inclined to get new injectors or even hit up a JY and get some used OEM ones tbh.


Sent from my iPhone using Tapatalk

Click to expand...

yeah i opted for a reman to test the theory. nothing in the parts bin looked worth the effort.

this kid was a moron, he left a solid 20$ worth of hardware out when putting it back together. I'm glad i ended up replacing the motor because i unscrambled a lot of eggs along the way

too clean of a 93 to let it get sold for parts/scrap

adsm08 said:

I have had very good luck with eBay injectors. Both my 4.0s are running full sets of Bosch remans bought from eBay.

Click to expand...

lol yeah bosch remans... this was a bag of brand new cheap chinaman jobbies that wouldnt fit.

That thing looks pretty straight

FlipFlopFabworx said:

lol yeah bosch remans... this was a bag of brand new cheap chinaman jobbies that wouldnt fit.

Click to expand...

I don't care who made them. Both engines straightened up and idled much smoother after the new injectors were installed. And they have been working fine for 3 and 5 years respectively.

adsm08 said:

I don't care who made them. Both engines straightened up and idled much smoother after the new injectors were installed. And they have been working fine for 3 and 5 years respectively.

Click to expand...

Yeah if they’re a reputable brand I have no problem buying parts off ebay. That’s how I get a lot of my OEM parts for my vehicles and dirt bikes. I just figured he meant like some china junk eBay brand “injection masters” or some BS hahahah [emoji23]


Sent from my iPhone using Tapatalk

so after a few weeks it seems to be the injector, i replaced the suspect one and it helped out a lot. it still idles a little funny but it seems to have gained the power and fuel mileage back it was lacking. No more check engine lights. hopefully the last of the idle hiccups will iron out when i replace the rest.

Thanks for the help!