91 bronco stalling and hard shift
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From: warrenton mo
91 bronco stalling and hard shift
Just got this 91 Bronco yesterday. The truck is stalling after it runs for a few minutes. It also has an EXREMEMLY hard upshift. It fires right up with a tap of the gas even after stall. Pulled IAC and it seems to be functioning. Ran KOEO codes and pulled code 126, 327 and 328. This is the first time I've ever used this type of scanner. It has a broken vaccuum line for the map sensor that I plan to replace tomorrow. Truck has fresh plugs, wires, cap and rotor and fuel filter. Havent checked fuel pressure yet but I can hear the relays working and pump running. Pulled wires to the EGR and truck idle seems to smooth out but havent test driven. It runs smoth and pulls strong then as soon as it warms up it stalls out. also have to give it gas to start cold. checked fpr also it was good.It only has 44,xxx miles on it. Any suggestions on where to start would be greatly appreciated.
PS Love the site, was a huge help in building my 89 xcab longbed 4x4 5 speed big block F150!!!!!
PS Love the site, was a huge help in building my 89 xcab longbed 4x4 5 speed big block F150!!!!!
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From: Gateway to the West
Codes 126, 327 and 328 are related by two common denominators; electronics and vacuum. Code 126 can be triggered by either a lack of vacuum to the MAP or a bad MAP. Both have pretty clear-cut solutions. Code 327 indicates a lower-than-expected voltage reading which either means that the pintle in the EGR valve is not seating fully when its closed (causing inaccurate readings from the EVP sensor) or a bad EVP sensor. Code 328 indicates the SAME problem but also indicates that the voltage was low even with the valve completely closed. Again, this could either indicate a bad EVP or a stuck EGR valve. Either way cleaning the EGR and making sure it fully closes would be the first step in troubleshooting. (DON'T use solvents to clean the EGR valve)!
There is an extremely unlikely possibility that the VREF (reference voltage from the ECM) is too low to begin with but this would typically affect ALL of the onboard sensors except the HEGO and knock sensors.
There is an extremely unlikely possibility that the VREF (reference voltage from the ECM) is too low to begin with but this would typically affect ALL of the onboard sensors except the HEGO and knock sensors.
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From: Gateway to the West
Not true. The ECM will compensate for a lack of information from any one sensor and if it gets too complicated to compensate, a properly functioning ECM will fall into "limp mode" which automatically applies a set of pre-programmed limited parameters that will allow the vehicle to be driven at moderate speeds with very limited performance. This keeps you from being completely stranded if a single sensor fails.
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Uhm, from my experience with 3 different map sensored fords, when the map sensor fails or there is no vacuum to it, that limp mode is 25mph with the throttle smashed to the floor. Would you really consider that working correctly? that's what I was getting at. I'll bet the farm that fixing that vacuum line will solve most, if not all his problems.
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From: warrenton mo
Thanks for the input! I suspected the egr as the culprit. I will pull it later today and clean and inspect for proper operation. As the previous owner of numerous blue ovals over the years I know they are picky about vaccuum. However i have never experianced these types of symptoms. At least to this extreme. This is also my first totaly bone stock bronco!!! will post later the results of my inspection/repair. hopefully ol poppa smurf responds positively!!!! 
Info I left out. 5.0, E4od.
Info I left out. 5.0, E4od.
Last edited by smurf91; Apr 16, 2012 at 09:07 AM. Reason: left out info.
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The map sensor tells the computer "hey, this is how much air pressure I have in the manifold." Which in conjunction with a throttle position sensor of sorts says "we should totally inject this much fuel at this time and at this rate!" It's like if you unplugged the maf on a mustang, yeah it will run, but it will run like poop!
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From: warrenton mo
Ok,here is the situation now. I pulled the egr, iac cleaned and inspected. Both working as far as i can tell. pulled more codes, koeo 126. koer 126 & 998. test drove the truck and it runs much better!! still have to hit the gas to start, still wants to stall. my question is, does code 126 indacate that the map has failed? trans still shifts hard and is slow to down shift. It makes sense the map works in conjuction with tpi. As far as i can tell the trans wont shift correctly if the tpi isnt sending the proper signal. And is code 998 indicating system limp moad? the vaccuum line that was broken ran to evap canister.Also found a wire cut on the left inner fender.The wire is red and blue runs to a green plug and appears to run down to the trans. I repaired it and couldnt tell any differance. Im now wondering if the wire shorted something out in the trans? again,thanks for the help!!!
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From: Gateway to the West
Uhm, from my experience with 3 different map sensored fords, when the map sensor fails or there is no vacuum to it, that limp mode is 25mph with the throttle smashed to the floor. Would you really consider that working correctly? that's what I was getting at. I'll bet the farm that fixing that vacuum line will solve most, if not all his problems.
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Its STILL RUNNING and its called "limp mode" for an F-ing reason! Its still working correctly because it is limiting engine revolutions and transmission shift points to avoid DAMAGING the engine, the transmission and a host of OTHER components that COULD be damaged if it DIDN'T! So, yes, 25 MPH with the pedal mashed to the FLOOR is perfectly acceptable when the onboard computer is keeping YOU from screwing up your own truck! In short, that's how its SUPPOSED to work! Back off unless you KNOW how the system works!
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From: warrenton mo
WOW!! Not quite sure what provoked that response from gray streak. If that was directed towards me, I dont fully understand how the componets work in conjuction and relate to eachother. Im not a compleate idiot on the subject but more of an apprentice. Thats why I asked all the questions. Im confused over the symptoms. 
Sorry gray streak! just went back over your first responce and saw that infact you had already answered my question about the map!! Chalk that one up to lack of sleep!! All work and no play = very dull day!
Sorry gray streak! just went back over your first responce and saw that infact you had already answered my question about the map!! Chalk that one up to lack of sleep!! All work and no play = very dull day!
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From: warrenton mo
Ok, I replaced the map and truck runs good. No more stalling and shifts smooth. Still has slightly high idol and shifts to soon. Have to give it a good stab of the throttle to get it to shift into a lower gear to pick up speed. no more coads and no check engine light. the speedo is a little off. does the speed sensor have anything to do with shift points? I think the throttle linkage needing some lube is cousing high idol.