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Old 10-31-2010, 09:33 PM   #1
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91 5.0 Idle / Low RPM Stumble & Surge w/codes

Working on a 91 Mustang 5.0. After the car is completely warmed up, I have a hard stumble off-idle. Also stumbles when revved to ~2000 rpm in neutral. Runs great when cold and while warming up, problem starts after 10-15 minutes. Seems to run OK above 2k rpm, but I haven't had much chance to really "enjoy" it yet. I just bought the car a few weeks ago, it was running VERY poorly at the time. I've made progress, but need some help now.

KOEO - code 11 - pass

KOER - codes 21 42 92

KOER was after a 30 minute drive with <2 minutes cooldown. The car has a new ECT sensor, and a 192* T-stat. It was completely up to temperature... O2 sensors are both new as well, since they were mismatched and one LOOKED really bad when I purchased the car (chunks falling out of it). For kicks, I took the old ECT sensor tonight and tested the resistance at room temp and at ~200* - it ohm'd out within range at both temps, so probably wasn't bad... Previous owner said he was getting 21 & 91 codes. Car smells very rich (as expected).

What do I check next??? Are there more tests for the ECT circuit? KISS please, I'm a relative newbie to all this electrical/EEC stuff (but not to cars)... THANKS - in advance...

A little more background info:
1991. Cobra intake, 24 lb injectors, 70mm throttle body, C&L elbow and maf, with correct blue tube, stock airbox. Unknown cam (sounds like a B303 to me), roller rockers, unknown heads, aftermarket shorty headers, Offroad H pipe (no cats).
Egr is functional, TAB & TAD solenoids are present, but airpump, hoses, and valves are gone. The previous owner had hooked the vacuum line from the TAB to the front of the intake - I have removed and plugged that line and plugged the charcoal canister back into that port. Vacuum line off TAD solenoid is plugged as well. I also removed the open breather and put a proper oil cap back on. I replaced the 160 T-stat with a 192, as the temp gauge barely moved before - now runs mid-gauge - Those fixes made the car run SO much better.... I've check pretty well for vacuum leaks, using a mirror to look underneath the upper. Right now, I have an occasional slight (but livable) idle surge, that gets pretty bad when the stumble shows up. When purchased, the car surged so bad all the time it would barely stay running hot or cold.
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Old 10-31-2010, 10:23 PM   #2
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Check the wiring to the ECT. See if you have 5v between the two wires.
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Old 10-31-2010, 11:07 PM   #3
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Just measured 4.53v between the two wires. Same 4.53v between the "hot" wire and engine ground. Checking the resistance between the other wire and battery terminal gave 20 ohms resistance.
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Old 11-01-2010, 10:48 AM   #4
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would probably run a lot better with the stock 19lb injectors and matching maf.. most likely still has the stock heads on there. just sayin
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Old 11-01-2010, 11:26 AM   #5
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Quote:
Originally Posted by smokeshow93 View Post
would probably run a lot better with the stock 19lb injectors and matching maf.. most likely still has the stock heads on there. just sayin
I don't think you can ASSUME anything on this car. Engine mods have been in place for a while, if the amount of undisturbed dust, dirt & grime on everything is to be believed. I tend to think this was someone's drag toy, as evidenced by the holes from a nitrous bottle mount and the Moser 9" rearend under it... I wouldn't take any bets on what's in the heads or shortblock.

Now, back to resolving to code 21... My plan is to back-probe the ECT connector tonight and record voltage readings from cold startup to operating temp. Sound reasonable liljoe07?
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Old 11-01-2010, 08:21 PM   #6
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First off clean the 10 pin connectors behind the intake. The voltage at the ECT is in range. Re-run the codes with engine fully warmed up, and see if that clears your code 21. May even clear the rich codes also.


If not.........

Take another reading at the ECT sensor key on engine off. Then go to the computer and take a voltage reading between PIN 7 and 46. It should be real close to what you got directly off the ECT. If not then you have a wiring issue or connection issue that needs to be found.
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Old 11-01-2010, 09:05 PM   #7
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Will pull the 10 pins apart tomorrow night and clean/tweak/grease.

Took the car for a drive tonight. Ran about 10 minutes with no issues, got up to temp after about 7 minutes. Shut it off for 30 minutes to eat a quick dinner. Came back out, it started right up, but ran very poorly - stumbled badly off-idle, like it's been doing after 20-25 minute runs. Ran it for 20 more minutes that way - if I kept rpms <1500 or over 3000 rpm, it tended to run better - around 2k was horrible.

Limped him and immediately hooked up the reader. KOEO was clear. KOER gave 21 and 42, no 92 this time. A quick check with the meter gave .63v at the ECT sensor. Shut the car off, pulled the connector and checked the sensor at 3.2k ohms... So I think you're onto something with the ten pins and rechecking at the computer. Sensor seems to check OK.

This thing REALLY acts like an ignition miss to me. RPMs instantly drop 400-500 rpm intermittently when things are warm. Can ten-pin wiring issues cause that, or am I chasing multiple problems (like a failing TFI module or ?)? I threw my timing light on, and the strobe seemed to "miss" as well...

I did pull the plugs tonight just for a look-see. Looked fairly fresh, and not as dark as I would have expected. AP25's. A couple seemed to be a bit loose to me. Gaps were reasonably consistent at .040 to .048 - I put them all back in at .044-.045. Plug wires look decent,but not fresh - Bosch Premiums. The cap & rotor do have brass contacts...
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Old 11-01-2010, 09:58 PM   #8
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10 pins are very problematic in some cars. First and foremost get all the codes squared away. Then move on to any other issues. Makes it easier when you don't have some sensor complicating matters worse.

Then once that is squared away. Run a cylinder balance test back to back. See if it pin points any problem cylinder.

Good Luck!
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Old 11-01-2010, 10:20 PM   #9
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I agree with your approach - let's solve one problem at a time, starting with the codes. Will do those 10-pins tomorrow evening!

Thanks for your help so far!
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Old 11-01-2010, 10:30 PM   #10
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No problem buddy!
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Old 11-02-2010, 07:16 PM   #11
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Took the 10 pins apart, cleaned, tweaked & greased. When I pulled the kick panel - I found a Superchips 9075-A module (this car is full of surprises!). I went ahead and pulled it out for now. One of those 2 things eliminated the code 21.

KOEO - code 11 -pass!

Continuous Memory - pass!

KOER - codes 42 & 92

Went ahead with the Cylinder Balance - pass!

Once again, the longer the car ran, the worse the skip/miss/cutout became... But, at least we're down to only "rich" codes. What's next?

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Old 11-03-2010, 11:24 PM   #12
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Check the fuel pressure first. Should be 39-40psi with the vacuum line going to the regulator disconnected.
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Old 11-04-2010, 10:45 AM   #13
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I'll have to get my hands on a fuel pressure gauge.

I did (temporarily) disconnect & check the vacuum line from the regulator last night- no gas inside it, and attaching a vacuum pump for 60 seconds gave no noticable leakdown on the vacuum side. The car does have a stock regulator. (It is re-connected)

----------------------------

I DID have a successful 45 minute drive to the DMV this morning..... Not sure if I fixed anything, or the 50* "cool" temps helped. No miss, no cutout, no skip. Here's what's different:

1) The TFI module was only attached with one screw. 2nd (front) screw is broken off flush with the housing, leaving a 1/8" gap. I "redneck engineered" a small vice-grip to clamp it back to the housing tightly.That MIGHT possibly explain the heat-related aspect of the problem... (Like I said, this car is FULL of surprises). I'll try to locate a spare screw, and see what I can do about getting the broken one out...

2) Entire distributor was loose - easily turned... Pulled the spout, set timing at 10* and tightened the clamp - no movement.

3) Reluctor wheel inside the distributor has noticable wear/rubbing on the insides of the "teeth". I was able to move the sensor just a tiny bit, so the wheel no longer rubs. I also blew out all the metal shavings from inside the dizzy. Shaft has noticable sideplay - not good (I think).

4) I went ahead and unplugged/capped the charcoal canister line at the manifold. Hard telling if there is a problem there, as it was previously not hooked up, and the vent line from the tank was cut as well. Just wanted to eliminate THAT variable for the moment...

So, the car ran flawless this morning. I plan to pull codes again this weekend and see where we're at. I'll try to get a FP gauge on it as well. It SEEMS like progress!!!

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Old 11-21-2010, 02:37 PM   #14
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Just to end the story on this...
A bad distributor was causing the miss, which led to the rich codes from both banks. There were a couple major issues with it - 1)the TFI module was held on by 1 screw, the other was broken off, and 2) Entire distributor was severely worn - shaft had tons of sideplay, causing the wheel to rub the sensor, and timing to jump all over the place. Put a different, "tight", distributor in, and the car runs flawless, with no codes!!!

Thanks, liljoe07, for your help in sorting this thing out!

Wear on the shutter wheel:


Distributor shaft slop - note the side-to-side movement:

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1991 Saleen #91-039 - SOLD 10/12
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