Hey guys,
New to the site and I have a couple questions.
I have a 1990 F150 with a 351 and a AOD tranny and installing new exhaust and headers. I have removed the stock manifolds, smog pump, everything connecting to the pump and the EGR valve. I have plugged the back of the heads where the emissions connected.
Is there going to be any problems with the Speed Density computer or not and is there any thing else that I need to do?

Thanks Eric

 

Anything that alters the volumetric efficiency of the motor (<-headers do this) will throw off the speed density system. In my experience, SD is very sensitive and it does not take much to throw it off. I have heard of other people get away with a few things though. The only way to tell for sure is to try it out and hook up an exhaust gas analyzer. If the lambda is off, you will have to either reprogram the EEC, convert to MAS or go back to a stock set up. If lambda is right on, then you are OK.

If you remove the EGR system you will have to get a resistor to plug into the place of the EGR position sensor. If you don't, the EEC will not know that no EGR is flowing and it will then advance timing as though EGR is flowing. The result of the advanced timing and lack of EGR present in the combustion chambor is detonation...you don't want that. If you get the resistor, the EEC will know that EGR is not flowing and it will never advance the timing as it does when EGR flows.

 

Thanks,

I was curious as to what would happen. Where can I get a resistor?

 

I bought my resistor (and other things) from RJM Injection Technologies.

http://rjminjectiontech.com/?p=9

 

Thanks dude,
You're a life saver

 

All you got to do is put the EGR valve with sensor back on and connect it. Put a block off plate between the valve and intake and the computer won't know the difference.

The motor will run fine with the exhaust system.

 

Yeah that will work as long as no vacuum gets to the valve, but that aint the cleanest way to do it.

 

Sure it will, as long as the vacuum actuator is not leaking it'll operate and the computer won't know the difference.

 

Hmmm, if the EEC commands vacuum to the EGR via the actuator and the vacuum does build up in the EGR without leaking out somehow the diaphram goes up and contacts the position sensor telling the EEC that everything went as planned. With a block off plate installed the exhuast gas never makes it to the combustion chambor as the EEC thinks it does...and there lies the problem. The timing advances and there is no EGR in the combustion chambor and then comes detonation. With the resistor installed thus sending an "EGR closed" signal to the EEC, the EEC knows there is no EGR flowing and the timing never advances as it does in EGR mode and this is what you want to achieve. An EGR that never recieves vacuum meaning the position sensor never gets contacted by the diaphram (or anything else) would give the same result.

 

Well, how about this. When exhaust gas is recirculated to the intake it actually dilutes the air/fuel mixture, that whould make the motor more prone to detonation, but the computer is programmed to add fuel when the EGR is open.
I think the problem people experience with detonation that seems to coincide with an EGR code, is because there is a leak in the egr system and air is getting into the intake all the time, and particularly when it's not supposed to and the computer is not adding extra fuel to compensate. This makes the A/F ratio artificially lean and the motor will be prone to detonation.

I ran my 5.8 for a couple years with the EGR blocked off as I described, there was no detonation and no codes thrown.

 

Any hindsight from this experiment?? How'd the old beast run with and without?

 

Reintroducing exhaust gas into the combustion cycle lowers combustion temperatures by dilluting the AF mixture. All else being equal, that would make the motor LESS PRONE to detonation. When an EGR code is not registered, degrees of advance proportional to the actual position change of the EVP sensor are added to the timing tables to compensate for EGR opening, not fuel. If the EGR gases are blocked from entering the intake, but the EGR valve is set to operate so the EGR Valve Position (EVP) sensor related codes are not registered and no other EGR system related codes are registered (the EEC-IV logic does not allow the operation of the EGR system when an EGR failure code is present), and the system is operational (valve opening/closing portion) detonation will take place at part throttle due to the advance in the ignition timing tables as calculated by the EEC and the lack of the exhaust gases present in the combustion cycle. If the EVP sends a "closed" signal to the EEC at all times whether it be via a resistor or removing vacuum to the EGR valve then the ignition timing tables will remain unchanged, failure codes (soft) 33 and 34 will register and the EGR function will be disabled.

(Detonation=the premature spontaneous burning of a fuel–air mixture in an internal-combustion engine due to the high temperature of air compressed in a cylinder.)

Blocking off the exhaust gas from entering the intake does not disable the system as far as the EEC is concerned. If the system (EGR Valve Regulator (EVR) solenoid, EGR valve and EVP) is intact and operational no diagnostic failure code (hard or soft) will register. How do you know there was no detonation?

 

Eco, dont get to pridefull.

Conaski has much more experance, and respect here.

Everything he is saying is correct.

Multipul users have done the EGR block off plate here in this forum, and it works!

 

This right here is the flaw in your reasoning, it's exactly opposite of what really happens. A lean A/F mixture results in hotter combustion temperatures, not cooler.


Because I advanced the timing from base until it was audible, and then backed it off a couple degrees. 5 years later the motor is still running and has good compression. If there is any detonation that I can't hear, it's not enough to hurt anything.

BTW.. I percieved no difference in performance with or without EGR flow. The IAC valve needs more frequent cleaning with the EGR connected however.

 

EGR serves to lower combustion temperatures at part throttle cruise conditions. It is not used at idle or at wide open throttle. Lower combustion temperatures lower NOx emissions, which is why EGR valves were implemented. Lower combustion temperatures lower knock propensity, allowing more advanced timing, and since the exhaust gases displace (not dilute) some of the inlet air, less fuel is required when EGR is active. The highest combustion temperatures occur at slightly rich mixtures and cool as the mixture is enriched or enleaned from that point. Lean mixtures burn cooler, but are more susceptible to knock because the smaller amount of fuel is more easily ignited. Sorry guys, but Eco is right here.