1988 F250 351
I finally determined that the start/stall (5-10 times) before staying running was not due to the any relay, ground, PIP/distributor malfuntion but rather the IAC being told to increase the air flow whenever the truck is on. I know that the ECU is suppose to do this in a measured way in response to the various sensor inputs. In my case even though all the sensors are sending out correct readings, the IAC is opening up too much, allowing way too much air in, and causing a constant lean condition whether at start up or hot idle. The O2 sensor constantly shows lean. I get an 11 code KOEO and a 41 code KOER. When warm this shows up in a rough, surging idle around 850 RPM but with only 16 or so inches of vacuum. If I disconnect the IAC once the truck is warmed up, the truck runs smooth with 20" of vacuum although its at 450 RPM. Reconnect the IAC and the air rushes in. If I disconnect either the TPS or the ACT sensor it has the effect of stopping the excess air flow from the IAC but the truck doesn't run real well. There are no vacuum leaks and the ECU is new. So my last resort is to try the IAC adjusting kit and hope that manually controlling the IAC output will be effective. Some people have had success but I'm cautiously optimistic! I really want to enjoy this truck having put so much time and money into it. The truck (or maybe it's just the nature of the EFI IV system) seems evil! There are an awful lot of posts that speak to this condition.



Any thoughts on what to do or where to go from here welcome.

 

And how do you know it's doing this? Since it's a duty cycle you should be able to watch it getting opened and closed with a volt meter or preferably a scopemeter.

And if too much air were getting in you would NOT get a lean condition / stall / surge because it is metered air if it MAS and the MAP would detect it and increase the fuel injector pulse width if it's speed density.

How do you know you don't have vacuum leaks?

 

It's a speed density setup. Obviously my saying "no vacuum leaks" is only as good as all the testing I've done.

I replaced all the vacuum lines. I disconnected and plugged all plenum ports including the PCV port. EGR is blocked off. Upper manifold was removed a second time and reinstalled with gasket sealant. Lower manifold was originally installed with gasket sealant. I disconnected the thermactor air lines to eliminate the possibility of any upstream air affecting the O2 sensor.

I see 7.0 DCV on pin 21 (IAC) at the ECM with the Monitor. I have never seen anything above or below 7.0 DCV to indicate duty cycling. With the IAC hooked up (sucking too much air) the MAP shows 112Hz, the O2 is .004 DCV and there is no switching with the idle speed is 850 RPM. If I pull the IAC wire the MAP goes to 104Hz, the O2 will move to .88 DCV and no switching with the truck idling at 450 RPM. My hope is that the IAC adjusting kit will allow some control so the O2 will switch and be around .450 DCV.

If I had a unfound vacuum leak somewhere I would think I would be seeing a RPM value way above the 850 RPM that I now have at idle. I have intentionally created a vacuum leak by disconnecting a plugged port on the plenum. The idle rises as expected presumably because the MAP is responding with an increased pluse width. And why would the truck idle so smoothly at 450 RPM with the IAC out of the picture. Wouldn't an unfound vacuum leak raise that idle above 450 RPM as well?

The air that's coming in the IAC does not appear to be metered. I can trigger the decrease/increase in air flow by
unplugging/plugging either the IAT (improperly called ACT before) or the TPS connectors if this means anything.

 

A vacuum leak on the underside of the lower manifold will not cause an increase in RPM on a SD system...due to the content of the crankcase vapors I guess. It will cause the motor run like crap or stall depending on the size of the leak.

If your IAC is not switching then you need to find out why it's not switching and go from there...the adjuster is not the answer to that particular problem. Even if you do get that adjuster hooked up, how then are you going to get the computer to monitor idle speed by cycling the IAC?? That adjuster is made for a motor with a big cam that needs more air per duty cycle to idle...that way people arn't stuckmessing with the TB screw causing a mess of other problems. It has been a long time since I have tested an IAC, but you should be able to back probe the 2 wires at the IAC with the key on and engine off and see it switching....I did this with a volt meter, but I don't remember if I did this with ohms or volts...but whatever I did I saw it switching. There should also be an ohms reading of about 9 ohms across the terminals of the IAC.

 

Seriously, there would be no increase in RPM if there was a leak below the lower manifold. I guess anything is possible. If so what a b*tch! Under such a condition, why would the IAC be allowing in more air? As you said before, wouldn't the MAP be triggering an increased pulse width. These trucks are picky,picky,picky. I'll try backprobing the IAC for pulsing.

If I can stomach another manifold removal, what sealant would be best in that area? I used Permatex High Tack on the second application of the upper manifold last time. Whatever happens, it has got to work this time.

 

No, crankcase vapors leaking into the intake is NOT the same as "air" leaking into the intake. I would look into a smoke test of the crankcase.

So you said this is a new motor. Where did you get the motor? Is it ANY different than stock? If so, depending on what changed, you may be looking at a MAS conversion.

 

From Rhino Engines - Fairport NY. The only identifier I see on the invoice besides the vin# is Engine: L351W88T. Then it says 88-93 E,F series Bronco w/ dipstick. The only thing I could do is call them and see if they know any more. When did 351 engines begin to go to Mass Air?

 

Im not sure, early/mid 90s though. Even if you call them, who is to say they did not use the right cam or mislable the motor? With these kinds of problems and a new motor, I am very suspect that there is it's not SD compatable. I ran into this myself with my van.....the machine shop told me the motor would work with the factory engine management system and it turns out they were wrong...I had the surging idle thing. I went nuts trying to find the problem then I finally second guessed the motor (after replacing just about everything else) and thats when I found that the cam was the problem. I got lucky and ended up with one of the LAST mass air conversion kits from Ford through Summit. I am not saying this IS your problem, but this is usually what happens when there is a compatability problem with the motor and computer, and it's definetly worth looking into.

 

Something else I've observed that might pinpoint the problem. One of the (many) tests I've done was retesting the compression and doing a leak down on all cylinders. Both results came out great. However, when pulling the plugs I noticed that #6 had a distinctly different look on it. It was slightly shiny with what looked like oil in the threads. Also the truck was loosing 1 quart of oil in 600 miles. There is no sign of oil leakage on the ground or on any other part of the engine. I optimistically was hoping that this was a syptom of the engine not being fully broken in (Old Paint had said his engine lost 1 quart every 1000 miles for about 5000 miles then it abated). After what you have stated about the possibility of a lower manifold leak this is starting to add up. I'm going to pull the #6 plug this morning and reassess.

 

Screw my optimism. Just pulled the plugs on #5 and #6 cylinder. #5 is perfect, #6 is crap - covered with oil with fresh oil all over the threads. Checked the oil level in the crankcase - it's down another quart in 500 miles. The compression and leak down percentage on #5 is fabulous. It must be the lower manifold pulling oil laden crankcase air into the intake. What a B*tch. On the bright side I found the leak (thanks ECO for your insight). On the down side its 19 degrees unlike the temperatures this summer when I replaced the engine. I'll just fire up the Reddi-Heater and bite the bullet.