Occasional rough idle - 5.0 SEFI

[toddz69]

I'm having an occasional idle quality issue on my '91 SEFI 5.0 engine for which I'm hoping someone can provide some assistance.

Engine background:

1991 5.0 HO SEFI engine from Mustang GT. Installed in '97 with 50K on the clock. Now has approximately 150K on it. Other than Explorer front dress, Explorer upper/lower intake (and corresponding EGR delete), TAB/TAD/smog pump delete, and headers, the engine is stock. Engine has run perfectly for over 11 years.

In October, I noticed upon cold start that after about minute or two of idling, the engine would idle roughly (not stall) for about 30 sec. to a minute. If I happened to be driving it at the time, giving it some throttle cleared things up shortly and it would return to its normal smooth idle by the next stop sign or stop light. If it was sitting in the driveway idling, the rough idle would happen for about 30 sec. - 1 minute and then it would smooth out again without any input from me. During one time when it was idling rough, I had a fuel pressure gauge hooked to it and the fuel pressure remained steady.

I pulled codes and came up with the following:

KOEO:

85 canister purge circuit failure (expected since I don't have canister purge)
33 EGR not opening (expected since I have RJM EGR eliminator)
14 PIP circuit failure
95 fuel pump secondary circuit failure

KOER:

12 RPM did not reach upper test limit
94 thermactor air failure (expected since I don't have TAD/TAB solenoids)
44 air pump system inoperative (expected since I don't have smog pump)
33 EGR not opening (expected since I have RJM EGR eliminator)

Curiously, my tester kept indicating it was testing a 4 cyl engine when I was pulling these codes.

I disconnected the battery to clear the PCM memory and cleared the codes from my tester and continued to drive the truck for several months.

In the past month, the rough idle at cold start problem has disappeared and now it manifests itself after the truck has reached operating temperature and is parked for 5-10 minutes (or more) and then restarted - essentially when the engine is in a "heat soak" condition. Again, it runs rough for about 30 sec - 1 minute after about 30 sec after you start it up and then soon clears up if you get on the throttle.

Two weeks ago, I checked the codes again and got the following:

KOEO:

85 - canister purge
33 - EGR

No PIP or fuel pump circuit failure codes this time.

KOER:

12 RPM test limit
92 Left EGO or HEGO was always rich (a new code this time)
33 EGR

It passed the cylinder balance test.

I was surprised not to have the usual TAB/TAD/air pump codes. This time my tester said it was testing an 8 cylinder engine .

Base timing is 12 deg. BTDC (I've run it there since I installed the engine 15 years ago)
TPS voltage is 0.78V. I thought this was of concern until I read this article:
http://www.stangnet.com/mustang-forums/threads/adjusting-your-tps-to-0-98v-is-not-necessary.825424/ and others claiming the 0.98-0.99V setting is not required.
I've cleaned the IAC numerous times (total disassembly) and the throttle body is clean as well.
Fuel pressure is a consistent 34-35 psi.

Any insights or suggestions welcome!

Thanks,
Todd Z.

 

[Viperwolf1]

Did you try cleaning the MAF? That has a huge effect on startup A/F ratio. Might be cause of code 92 also. Code 12 might be generated if you didn't goose the throttle during the KOER test.

 

[toddz69]

Did you try cleaning the MAF? That has a huge effect on startup A/F ratio. Might be cause of code 92 also. Code 12 might be generated if you didn't goose the throttle during the KOER test.

Don't think I've ever cleaned the MAF - CRC makes a spray for that too, don't they?

I do remember goosing the throttle the last time I did the KOER test...

I'll try the MAF spray next.

Thanks,
Todd Z.

 

[ransil]

maybe getting corrosion on electrical contacts.
get something to clean/treat the contacts.
I would start at the computer then the MAF, TFI TPS, O2 sensors

Also check grounds 11 years ago they were good.
I would also throw in check for a vacuum leak , but look like the different codes are pointing to electrical.

I use Kopr-Shield applied with a tooth pick
http://www.tessco.com/products/displayProductInfo.do?sku=437469&eventPage=1

or something like Deoxit

http://www.caig.com/

 

[Socal Tom]

Are you using single pump or dual pump/ accumulator set up? hot fuel in the accumulator or in a filter in front of the hp pump could cause this. fuel can boil in the filter and then the gas bubbles can get circulated through the fuel rail until everything cools down.
Btw i had similar issues that resolved when i removed the hegos, so you might also look at your hegos, they maybe tired and taking lomger to get to operating temp.
Tom

 

[toddz69]

Thanks for all the suggestions. Randy - I have some Deoxit in the garage - I'll apply that to some of the connections on the various sensors. I checked the computer ground a few months ago and made sure it was good. I'll look at the others as well.

Tom - were you still able to pass emissions when your hegos were starting to wear out? I had one fail years ago and the only clue I had was that I failed emissions rather spectacularly. I have the single pump setup for my system - I changed the filter as a precautionary measure as soon as I noticed I had a problem just to make sure it wasn't a possibility.

Todd Z.

 

[Socal Tom]

I was emissions exempt, so I never had it tested ( 67 and prior out of state are "not emissions controled"). I think my problem had to do with the hegos being way down in the collector on my headers. Smelled really bad with the hegos hooked up.
Tom

 

[toddz69]

Bringing this thread back to life. I was distracted for a few months fixing a few other items on the truck and finally had a chance to make progress on this issue. I got the CRC mass air sensor cleaner and cleaned up the MAF real well. I also checked and verified my grounds and made sure they were in good shape. None of those items made a difference.

Last week I finally had a chance to get a new HEGO for the driver's (left) side to try and correct the '92' code listed above. I haven't driven the truck much since then but it appears the problem *might* be gone (fingers crossed) and when I checked codes a few days ago, the 92 was not on the list.

Could a bad HEGO have caused the symptoms I listed above?

Todd Z.

 

[ransil]

HEGO should not have any affect on cold start, maybe a hot start.

 

[toddz69]

HEGO should not have any affect on cold start, maybe a hot start.

Yes, my symptoms have always occurred at hot start as of late.

Todd Z.

 

[70_Steve]

Could a bad HEGO have caused the symptoms I listed above?

Yes, my symptoms always occurred at hot start.

My experience with O2 sensors in industrial applications shows that the sensors get very sluggish to respond when they get old.

Also, you might want to check the heater circuit on the old O2 sensor, if you still have it. The sensors don't respond hardly at all until they reach about 800° or so. The built in heater gets them to that temp really quickly, but if the heater's not working it would take a good minute or two for the sensor to come back up to temperature, even on a warm engine.

 

[toddz69]

My experience with O2 sensors in industrial applications shows that the sensors get very sluggish to respond when they get old.

Also, you might want to check the heater circuit on the old O2 sensor, if you still have it. The sensors don't respond hardly at all until they reach about 800° or so. The built in heater gets them to that temp really quickly, but if the heater's not working it would take a good minute or two for the sensor to come back up to temperature, even on a warm engine.

Thanks for the info, Steve. Unfortunately I believe the sensor is probably under a bunch of palm tree trimmings in my trash can right now. I thought about testing it but opted for the "change it and throw it in the trash" option instead .

Todd Z.