EFI idle issue.....comes and goes

Timmy390 · Feb 6, 2017

Sorry long.....

351W GT40 upper and lower intakes GT40P heads SN95 EFI with 96 Explorer MAS, TPS and IAC Issue just started a few weeks ago and is getting worse.

The rig was running great with no issues. I stopped to get gas and when I restarted the engine, the idle surged to 1500 and then dropped to 300 then surged again to 1500 then the rig died. I restarted it and the same thing. Restart again and everything was fine. Drove home no issues through several stop lights.

A few days later, same thing, stopped at the parts store and when I came back out, surging idle and dying. Got it going and at next stop (a red light) everything was back to normal. Cleaned the IAC (it wasn't dirty) and installed new gasket.

Dove fine for a few trips no issues

Now, the issue is getting worse. It was only doing this after a warm restart. Now it's doing it (surge not as bad) on cold startups. The issue comes and goes but is getting more frequent.

Check engine light popped over the weekend but then went out before I got home. Pulled code 173 (running rich).

I pulled codes again on Sunday (CEL not on) and only got 452 (VSS not working properly) so I unpluged it. Rig drove the same as it did before with the idle issue coming and going. No other codes.

The issue is accompanied with bucking when ideling along in slow traffic.

Any thoughts on where to start? Kind of ruled out a vacuum leak as it comes and goes. Thinking TPS but don't want to just throw $30 and $50 parts at it.

Tim

needabronco · Feb 6, 2017

A few things come to mind. Check the vacuum caps on the bottom of the upper intake, I chased an intermittent idle issue for longer than I'd like to admit and I had a cracked vacuum cap. Second make sure all of your injector plugs are plugged in and tight, I have 1 that likes to loosen up every once in awhile. Lastly, take your IAC apart and clean it out and make sure its not sticking. If you've mounted it backwards for hood clearance make sure you switched the wires in the plug.

ScanmanSteven · Feb 6, 2017

There is a procedure for setting the base idle, did you do that? I'll try to find it but it's for the earlier ecm, not sure that makes any difference. Also, make sure both o2 sensors are ok, not laying on the manifold or burnt wires.

Viperwolf1 · Feb 6, 2017

Make sure you have the O2 sensors connected to the correct side of the harness.

Timmy390 · Feb 6, 2017

Thanks guys

needabronco said:
A few things come to mind. Check the vacuum caps on the bottom of the upper intake, I chased an intermittent idle issue for longer than I'd like to admit and I had a cracked vacuum cap. Second make sure all of your injector plugs are plugged in and tight, I have 1 that likes to loosen up every once in awhile. Lastly, take your IAC apart and clean it out and make sure its not sticking. If you've mounted it backwards for hood clearance make sure you switched the wires in the plug.

The IAC is mounted to the TB in it's factory spot/orientation plug pointed down as was from the factory. The TB is rotated 90 degrees clockwise. So rather than being on top, it's on the side now with TB linkage on the bottom. I took it off and cleaned it....how do you take them "apart". It didn't look like it be serviceable. I'll look again at the vacuum caps. Injectors are all plugged in tight.

ScanmanSteven said:
There is a procedure for setting the base idle, did you do that? I'll try to find it but it's for the earlier ecm, not sure that makes any difference. Also, make sure both o2 sensors are ok, not laying on the manifold or burnt wires.

I haven't done any setting of idle. It's always idled at 650-750 where EFI Guy set the ECU as base idle.

O2's are new, well 6 months old with maybe 150 miles on them. Already check for burned wires and or cracked.....

Viperwolf1 said:
Make sure you have the O2 sensors connected to the correct side of the harness.

Yes sir.....but I will double check that. I haven't had them disconnected since the build a year ago.

Tim

Viperwolf1 · Feb 6, 2017

New O2 sensors, you say? Make sure they are 3 wire sensors, not 4 wire. 4 wire sensors won't work in a 3 wire harness.

Timmy390 · Feb 6, 2017

Viperwolf1 said:
New O2 sensors, you say? Make sure they are 3 wire sensors, not 4 wire. 4 wire sensors won't work in a 3 wire harness.

Just looked and I have 4 wire O2's but 4 wire O2's were on the 94-95 Stang (running SN95 EFI) as well as the 96 van the harness is from. Same O2 was on the 96 Explorer I pulled the intake parts from. Van was speed density so I had to source another O2 harness but that too came from a 4 wire O2. There Bosch if that means anything.....I'll have to dig through and find the exact part number I ordered and received.

Tim

Viperwolf1 · Feb 6, 2017

Ok. I have a diagram that shows '94-'95 mustang as 3 wire.

Timmy390 · Feb 6, 2017

I'll dig up the part number and verify the part.....Going to take a bit.....it's in a mountain of stuff

Tim

Timmy390 · Feb 6, 2017

That was fast.....why it was just below top I'll never know....

95 Ford Mustang 5.0L V8 BS 15716 Oxygen (O2) Sensor 2 each 4 wires 2 white, 1 gray and 1 black

Ordered from Rock Auto....so who knows......Same part number as Oreilly's shows and the picture is what I have....

I did look at my SN95 diagram and it does appear 3 wire.....Very odd.

Tim

Timmy390 · Feb 6, 2017

OK, so did some verification today.....

Vacuum caps are all good....All connections/plugs are on correct and tight.

Figured out the TB, intake, IAC and MAS are from a 97 Explorer not a 96 as was written on the windshield of the junkyard donor. Figured this out due to the TPS sensor changed in 97 and I have a 97 sensor. Never trust a junkyard.....

Checked operation of TB and linkage and they operate smoothly. TB and IAC are clean. Return to idle does hit the stop screw on the TB

Now the odd thing......

Started and stop the engine a few times. On the last time as it was coming up to temp, the idle hung at 1000rpm vs the 650-750 it normally runs at. Turned it off and heard a loud hissing. It appeared to be coming from the brake booster. It went away before I could fully locate it.

On a whim I replaced the booster check valve with one I had in my tool box. Started it up and let it run for a few seconds, idle was back to 750ish and when I shut it off, the hiss wasn't there.

So could this be a leaking check valve? More likely a leaking booster? I wouldn't expect that kind of a leak to "come and go"?

Will drive it tomorrow and report back........I have to go help a buddy tare down a Northstar V8 tonight. Leaking head gaskets and what I think is a cracked block.....I think he's boned.....

Tim

Timmy390 · Feb 7, 2017

Not the brake booster. Plugged the vacuum line to the booster and drove it around. It was fine till I stopped for gas.

Started it after fill up, idle surged, dipped, surged and then dropped low and the engine died. Same exact thing 2 more times in a row.

On forth startup I just took off and pulled to a stop sign, Idle bounced then leveled out. after that, 3 stop light and 2 stop signs it idled fine. Pulled into the drive and parked....idle was fine.

No codes....

Tim

jw0747 · Feb 7, 2017

Go here...5 parts included. If you had the battery disconnected for any length of time your ECU needs to be reset...part 3. Pics shown are of '94-'95 intake.

http://www.muscularmustangs.com/iac.php

Timmy390 · Feb 7, 2017

Thanks for the info.

I played around with it tonight. Took it for a drive to get it warm and after tried to pull codes, KOEO 111 KOER 111

Funny thing happen, as I was testing it trying to get codes to pop with engine running, it got up to temp about 210 (I'm running a 195 stat) and it got to a point where it wouldn't stay running. It stays at or about 200 when I'm driving it around. I'm running an electric can so no air flow when just idling.

Start it, idle would hang at 1000 then drop to 700 then up to 1000 then 300 and die. It just wouldn't stay running. Start it back up, same exact thing. This was at 5:30 or so. It's just been sitting there since. After supper I went to pull it into the shop and it started and stayed running. The idle was bouncing about 200 rpm 900-700 but it ran. Engine temp down below 180.

It was running fine for the past few months EFI was. I did change heads but that wouldn't cause it to not idle with hot......

I'll keep digging......

Tim

Viperwolf1 · Feb 7, 2017

What is the fuel pressure doing during all this?

Timmy390 · Feb 7, 2017

Viperwolf1 said:
What is the fuel pressure doing during all this?

Good call on that as most of your calls are.

Not sure as of yet. Was talking to my buddy with the FUBAR Northstar V8 mentioned earlier and he ask the same thing. I had to search for my fuel pressure gauge. Found it just a few minutes ago. Have not used it in 7 years. Hope it still works......

I'll get some numbers and let you know.

Tim

EFI Guy · Feb 7, 2017

Make sure your fuel pressure regulator isn't leaking fuel into the vacuum line too, might explain the rich code.

surfer-b · Feb 8, 2017

If you have another coolant temp sensor swap it out just to try, I have had them do some strange things. I hate to have you throw parts at it without knowing but that's an swap.

Timmy390 · Feb 8, 2017

I have a spare FP regulator so I'm going to swap it out tonight which requires the intake to be removed. I'll get a better look at the vacuum line going to it. Also see if the intake gasket is OK. I'll also be checking the FP with my gauge cold and then when hot.

I was reading about the coolant temp sensor causing this kind of issue. It's on my list.....

Tim

EricLar80 · Feb 8, 2017

Your parts seem a bit mis-matched without a simple tune. Especially considering the 351w.

Eric

EFI idle issue.....comes and goes

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