chbtech
Member
- Joined
- December 20, 2006
- Messages
- 41
- Reaction score
- 0
- City, State
- Simcoe, Ontario
- Year, Model & Trim Level
- '87 Ranger
I drive an 87, 2.9, A4LD pickup....daily driver. The engine bogged (hesitated) every time you took it just off idle. You could even make it bog by moving the throttle while under the hood and in Park. It would catch in about 1 sec and then run normally. I tested/changed/cleaned many things incl the IAC, MAF, TPS, EGR, O2 Sensor, etc.
I set up the TPS to 0.95V output by adjusting the throttle stop screw. This had some effect +/- but didn't cure the problem. Then I got some advise that told me the bog was a timing problem. Pulled the spout and checked the timing...bang on 10* BTDC. Put the spout back in and whatched the timing advance quite nicely, so I knew there wasn't anything too far wrong with the initial timing or the advance. Knowing that the TPS effects timing, I gave that another shot.
The person who gave me the advise asked if the TPS was adjustable or not. Well I had never seen an adjustable one on a 2.9, but this got me thinking. I could never make sense out of the concept that to set the TPS voltage to spec, you are supposed to (I think) turn the throttle stop screw until the output voltage is on spec. However, what does not make sense to me about that concept, is that when you turn the screw say, to increase the voltage, you are "Opening" the throttle plate at the same time. To me, this would then confuse and defeat the purpose of the IAC by allowing air into the engine via the partially open throttle plate, when the throttle plate is supposed to be closed, and the air flow into the engine is supposed to be controlled by the IAC alone.
So I decided to "make" the TPS adjustable. I simply drilled out the 2 mounting holes so that the mounting screws had some play in the screw holes. Then I backed off the throttle stop screw so that the throttle plate was very close to being totally closed. (This of course would have changed the unmodified TPS output to 0.8V or so.....out of range). Then I remounted the TPS and with the throttle closed, adjusted it to 1.0V by rotating it on the screws. Bog is 95% gone and I'm a happy camper.
I talked to a mechanic about this and he said that the throttle stop screw is only there so that you can keep the throttle plate adjusted just slightly off the bore of the throttle body, so that the throttle plate doesn't jamb. I think that it also provides a means to close the throttle plate a bit if necessary to compensate for wear on the plate and the throttle body bore. He also said that some TPS are not adjustable because the computer compensates for the initial voltage output somehow. I don't follow that logic because the TPS is supposed to be adjusted to 0.9V to 1.0V output and the computer in my truck would not perform that adjustment. The only way to get that output voltage on my truck is to either adjust the throttle stop screw or make the TPS adjustable. I did the latter and it worked.
Any comments ??
I set up the TPS to 0.95V output by adjusting the throttle stop screw. This had some effect +/- but didn't cure the problem. Then I got some advise that told me the bog was a timing problem. Pulled the spout and checked the timing...bang on 10* BTDC. Put the spout back in and whatched the timing advance quite nicely, so I knew there wasn't anything too far wrong with the initial timing or the advance. Knowing that the TPS effects timing, I gave that another shot.
The person who gave me the advise asked if the TPS was adjustable or not. Well I had never seen an adjustable one on a 2.9, but this got me thinking. I could never make sense out of the concept that to set the TPS voltage to spec, you are supposed to (I think) turn the throttle stop screw until the output voltage is on spec. However, what does not make sense to me about that concept, is that when you turn the screw say, to increase the voltage, you are "Opening" the throttle plate at the same time. To me, this would then confuse and defeat the purpose of the IAC by allowing air into the engine via the partially open throttle plate, when the throttle plate is supposed to be closed, and the air flow into the engine is supposed to be controlled by the IAC alone.
So I decided to "make" the TPS adjustable. I simply drilled out the 2 mounting holes so that the mounting screws had some play in the screw holes. Then I backed off the throttle stop screw so that the throttle plate was very close to being totally closed. (This of course would have changed the unmodified TPS output to 0.8V or so.....out of range). Then I remounted the TPS and with the throttle closed, adjusted it to 1.0V by rotating it on the screws. Bog is 95% gone and I'm a happy camper.
I talked to a mechanic about this and he said that the throttle stop screw is only there so that you can keep the throttle plate adjusted just slightly off the bore of the throttle body, so that the throttle plate doesn't jamb. I think that it also provides a means to close the throttle plate a bit if necessary to compensate for wear on the plate and the throttle body bore. He also said that some TPS are not adjustable because the computer compensates for the initial voltage output somehow. I don't follow that logic because the TPS is supposed to be adjusted to 0.9V to 1.0V output and the computer in my truck would not perform that adjustment. The only way to get that output voltage on my truck is to either adjust the throttle stop screw or make the TPS adjustable. I did the latter and it worked.
Any comments ??
