Vintage Mustang Forums banner
  • Hey everyone! Enter your ride HERE to be a part JUNE's Ride of the Month Challenge!
  • May's Ride of the Month contest ended with a tie! Go to this thread to vote on the winner! VOTE HERE
1 - 18 of 18 Posts

·
Registered
Joined
·
4,971 Posts
Discussion Starter · #1 ·
What would cause a low idle RPM?

This problem seems to come and go, but more often than not, if I don't feather the throttle at startup a little bit, the engine will not stay running. The engine starts immediately with the turn of the key, but if I don't quickly apply some throttle, it'll immediately die.

After feathering the gas pedal I can get it to settle into a LOW idle RPM and it'll stay running. This idle is smooth, but still seems somewhere between WAY TOO LOW and JUST BARELY ENOUGH to keep the engine running smoothly.

The big annoyance is downshifting while driving... again, the throttle has to be feathered or it'll stall between slow downshifts (as if approaching a stop light).

If I'm sitting in Nuetral and rev the engine, throttle response is crisp and immediate, but again, if I take my foot off the pedal after revving, the engine will stall out.

About the only thing I could immediately think of was to double check the ignition timing. With the SPOUT disconnected, idle RPM increases and she stays running just fine. Timing with the SPOUT disconnected is perfectly at 10 degrees BTDC. After reconnecting the SPOUT I noticed that ignition timing jumps up to around 20 degrees BTDC and seems to move around slightly while the engine is idling (I'm lost why it would do this...?)

All of the codes except for 81, 82, 84 and 85 have cleared. The check engine light is not illuminated while the engine is running.

Manifold vacuum leak?

What else could it be?

I stopped by and talked to a trusted mechanic on the way home from work and he mentioned that the computer "had to adjust itself to it's new components in order to "set" itself"... I'm not sure what the heck he's talking about with that statement....

Does that make sense to any of you all?

Any ideas at all are more than welcome because my head hurts from thinking about it...

Dave
 

·
Registered
Joined
·
2,167 Posts
Dave,

I was having the same issues before I added the blower. I assumed it was because of the "learning curve". I also didn't even time the engine yet so that may be my cause. I was trying to time it when the tow truck arrived to take it to the body shop.

How are you checking the codes? Do you have a code reader?
 

·
Registered
Joined
·
302 Posts
If you're running a factory PCM setup you must use the idle setting procedure in the shop manual. If you just crank the throttle stop screw to set the idle speed, you are almost guaranteed to have problems. The idle speed is controlled by the PCM via the air bypass valve and the throttle plate must be set to a very specific range or it will not work properly. Idle surging and stalling will result.

If the idle is not stable with the throttle plate set as specified in the shop manual, you can adjust the idle speed using an air bypass plate or by drilling a bleed hole in the throttle plate.

The PCM sends spark advance information to the TFI module over the SPOUT circuit, which is why it must be disconnected when setting the timing. It is normal for the spark advance to wander at idle, it's just the PCM reacting to minute changes in engine RPM.

Jay
 

·
Registered
Joined
·
1,004 Posts
I'm guessing your using an A9L, or A9S EEc?

Anyway, stock idle is about 670 RPM. Is the ICS new or well cleaned?

Eric
 

·
Registered
Joined
·
4,971 Posts
Discussion Starter · #7 ·
use the idle setting procedure in the shop manual.
Yeah, I guess it may be time to break down and buy one of those...

Every sensor/component on the entire engine is brand new except for the A9L EEC and now that I think about it the IAB solenoid... and now that I really think about it... the inside port area of the solenoid looked awfully dirty/carbonized when I had it off...

Let me explore the IAB before jumping further. It could be a real simple fix.

Thanks!

Dave
 

·
Registered
Joined
·
302 Posts
Those two links are close although the first one was obviously written by someone unfamiliar with EEC systems, and the second one is missing alot of the procedure.

As for the computer learning, there are many things that are adjusted 'on the run' such as fuel, spark, high idle, base idle, etc. When the PCM is first powered up after losing its battery connection, it runs off of a base lookup table that is a part of its read-only memory. As the engine operates the PCM may find that it needs to add, say, 5% more IAC output or 3% extra fuel under certain conditions. If these needs are consistent, it will save these values in a new non-permanent look-up table that is stored in its 'Keep-Alive Memory' or 'KAM.' Under normal operating conditions it now uses these values as the base-line values and ignores the ROM. The KAM is constantly updated but changes are made in very small increments over time. The purpose of this is to account for production variances and to adjust for wear long-term. So, any time a component is replaced with a substantially different part or major adjustments are made, the KAM should be cleared by disconnecting the battery for a couple minutes to allow the PCM to make a new look-up table which will now include adjustments for the new parts.

Jay
 

·
Registered
Joined
·
4,971 Posts
Discussion Starter · #9 ·
As an update to this thread, upon getting home from work this evening I removed the Idle Air Bypass solenoid, cleaned it out with a can of carb cleaner (it wasn't really that dirty, but there was a light coat of carbon on everything inside that eventually came clean with repeated blasts from the spray can of carb cleaner), reinstalled it and took the car on a little test drive. I noticed the difference immediately as the car no longer stalled (pretty easy to notice!). I drove around testing the idle for approx. 15 minutes and alls well in paradise... well, at least this problem is fixed anyway...

Thanks for all of your assistance and advice!

Dave
 

·
Registered
Joined
·
780 Posts
Are you using a Speedometer sensor? That should address your problem during decelleration.

I had the same IAB problem early on. Swapping that, plus setting the TPS to .98v helped.

and it's true...you need to run it so it learns and smooths out it's calculation curves.
 

·
Registered
Joined
·
12,304 Posts
Yeah thats a good point. Do you have a VSS installed? Once again, I'm unfamiliar with your harness. My harness was an 88 speed density harness, so I've ran some wires from the VSS up to the computer harness.. Once again, this is all untested..

You're going to have the bugs all ironed out for me by the time I fire it up.. :lol: I MIGHT have another VSS around here somewhere potentially..
 

·
Registered
Joined
·
4,971 Posts
Discussion Starter · #12 ·
Yes, the VSS is installed. Knocking on wood... I believe the low idle problem is solved.

Now I've got the rearend torn back apart....

Dave
 

·
Registered
Joined
·
2,167 Posts

·
Registered
Joined
·
272 Posts
I am having the exact same problem on my new 302 EFI install. I'll try cleaning the idle solenoid.
 

·
Registered
Joined
·
1,937 Posts
I own several 5.0 Fox bodies, I found a great site to help troubleshoot these type of issues:
SBFtech.com You need to register to see all the forums. There is a guy on that site named Joel who seems to know a ton about 5.0's. Post your question, he'll ask you to run a code scan and then will post a lot of useful info.
 
1 - 18 of 18 Posts
Top