Engine racing on its own
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Engine racing on its own
Hi Folks,
'94 Mazda B-3000 4x4 manual tranny. 3.0 litre engine.
i'm having a strange issue that seems to happen intermittently. Will be driving and i will notice that it seems like the engine is requiring me to give it less throttle to maintain the same speed. If i depress the clutch, the engine will race to around 3400 rpm... and keep on racing like that for a while, before it eventually drops back down, even though i am not depressing the throttle. The strange thing is that if i pull over, with the clutch depressed shut down the engine and just restart... then the truck behaves normally with proper idle etc.
The only thing i can think of would be a massive vacuum leak (checked for it... nothing obvious but i can recheck), the idle air valve motor somehow getting stuck open, or the throttle or throttle cable getting stuck (nope all fine there).
The time it takes for the engine speed to drop back down varies... but eventually it does drop back down to a slightly more than normal idle. When it is in this mode.. the idle is slightly higher than normal (its almost like the truck idles at the same speed as when the truck has just been started up cold in open loop prior to it going into closed loop, when the idle drops slightly).
Its almost like the motor is going into a different mode, and once it goes into this mode it needs to be shut down to get out of it.
Anyone have any similar experiences with their Rangers?
'94 Mazda B-3000 4x4 manual tranny. 3.0 litre engine.
i'm having a strange issue that seems to happen intermittently. Will be driving and i will notice that it seems like the engine is requiring me to give it less throttle to maintain the same speed. If i depress the clutch, the engine will race to around 3400 rpm... and keep on racing like that for a while, before it eventually drops back down, even though i am not depressing the throttle. The strange thing is that if i pull over, with the clutch depressed shut down the engine and just restart... then the truck behaves normally with proper idle etc.
The only thing i can think of would be a massive vacuum leak (checked for it... nothing obvious but i can recheck), the idle air valve motor somehow getting stuck open, or the throttle or throttle cable getting stuck (nope all fine there).
The time it takes for the engine speed to drop back down varies... but eventually it does drop back down to a slightly more than normal idle. When it is in this mode.. the idle is slightly higher than normal (its almost like the truck idles at the same speed as when the truck has just been started up cold in open loop prior to it going into closed loop, when the idle drops slightly).
Its almost like the motor is going into a different mode, and once it goes into this mode it needs to be shut down to get out of it.
Anyone have any similar experiences with their Rangers?
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From: SW Va
Welcome to FTE.
Good trouble shooting for the usual suspects.
Scan for trouble code clues & post All code Numbers, even pending codes if no CEL has been lit, or if it has been lit but now is off..
The idle on the 99 4.0L in my signature was suddenly acting out somewhat like this about 10 years ago & it turned out to be an acting out IAC. I had no trouble codes.
I could clean the IAC & it would be ok for a while, then just suddenly act out again by going at throttle up when I was trying to slow down for a stop. Interestingly It never did set a IAC trouble code. Pulling & cleaning the IAC with a plastic safe solvent like CRC MAF sensor spray & having that fix the problem for a while, was my trouble shooting clue to replace the IAC. I did so with a BWD brand from Advance Auto back when they were handing out 40% off coupons. Anyway the BWD IAC has been fine for the last 10 years & was Way less expensive than the Motorcraft one, which is also supposed to be a good replacement.
Some thoughts for consideration, let us know what you find.
Good trouble shooting for the usual suspects.
Scan for trouble code clues & post All code Numbers, even pending codes if no CEL has been lit, or if it has been lit but now is off..
The idle on the 99 4.0L in my signature was suddenly acting out somewhat like this about 10 years ago & it turned out to be an acting out IAC. I had no trouble codes.
I could clean the IAC & it would be ok for a while, then just suddenly act out again by going at throttle up when I was trying to slow down for a stop. Interestingly It never did set a IAC trouble code. Pulling & cleaning the IAC with a plastic safe solvent like CRC MAF sensor spray & having that fix the problem for a while, was my trouble shooting clue to replace the IAC. I did so with a BWD brand from Advance Auto back when they were handing out 40% off coupons. Anyway the BWD IAC has been fine for the last 10 years & was Way less expensive than the Motorcraft one, which is also supposed to be a good replacement.
Some thoughts for consideration, let us know what you find.
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hi,
just for a preliminary follow up... i ended up replacing the IAC with this one:
https://www.autozone.com/engine-mana...95736_232857_0
Evidently there are a few different IACs with different connectors coming out different ways, but this one is the proper one for the 94 Mazda B3000 with the cast aluminum intake manifold with throttle body built into the casting.
Immediately after replacing it... on the first drive... the engine went into a very high idle... 3500 rpm if i was coasting with the clutch pushed in... and 2200 rpm if stopped. i shut her down and after about 1 hour started her back up again... the idle was just fine. i am not sure if the computer just realized that its previous settings (for the old valve) were garbage and just discarded them and started over, but now it seems to be fine knock on wood. i did not disconnect battery when replacing the IAC.
One other detail... i noticed i was low on coolant (engine running a bit hot), so added coolant (now things are normal). Kind of makes me wonder if this is possibly coolant temp related as well. Low chance but it cannot hurt to mention it.
Will report back in a week or two. If the 3500 rpm idle while moving and with the clutch in does not recurr then i will consider this fixed. This has really been the only driveability issue i have with this truck.
just for a preliminary follow up... i ended up replacing the IAC with this one:
https://www.autozone.com/engine-mana...95736_232857_0
Evidently there are a few different IACs with different connectors coming out different ways, but this one is the proper one for the 94 Mazda B3000 with the cast aluminum intake manifold with throttle body built into the casting.
Immediately after replacing it... on the first drive... the engine went into a very high idle... 3500 rpm if i was coasting with the clutch pushed in... and 2200 rpm if stopped. i shut her down and after about 1 hour started her back up again... the idle was just fine. i am not sure if the computer just realized that its previous settings (for the old valve) were garbage and just discarded them and started over, but now it seems to be fine knock on wood. i did not disconnect battery when replacing the IAC.
One other detail... i noticed i was low on coolant (engine running a bit hot), so added coolant (now things are normal). Kind of makes me wonder if this is possibly coolant temp related as well. Low chance but it cannot hurt to mention it.
Will report back in a week or two. If the 3500 rpm idle while moving and with the clutch in does not recurr then i will consider this fixed. This has really been the only driveability issue i have with this truck.
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Joined: Jan 2004
Posts: 13,851
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From: SW Va
Ok, good feedback on replacing the IAC having finally fixed the random rpm runaway. When my rpm would run away like that I'd quickly shift to "N" & let the computer rpm limit take control of it & that would reset things for a while. It's a dangerous situation if your not prepared for it by before hand thinking about what to do if it happens. Mine did it randomly 3-4 times over many months before I finally zeroed in on the IAC.
If you had a trouble code set, or pending code stored but didn't erase it after replacing the IAC, the computer wouldn't have known you performed a repair, so may have mis-controlled things for a bit.
If the computer detected engine temp on the rise, it could have commanded the IAC to go at rpm up to add some additional fan cooling & coolant flow volume to check the temp rise.
I've not had a return of the runaway rpm since I replaced the IAC. My faulty IAC was the Factory OEM, so was another reason I chose the BWD brand, in case there was a OEM/Ford design problem, maybe I'd not get bit again. There haven't been many reports on this forum of this happening over the years, but in all cases I remember, replacing the IAC fixed it, so maybe you've put the fix on this puppy too!!!
More thoughts for consideration. Let us know how it goes over time.
If you had a trouble code set, or pending code stored but didn't erase it after replacing the IAC, the computer wouldn't have known you performed a repair, so may have mis-controlled things for a bit.
If the computer detected engine temp on the rise, it could have commanded the IAC to go at rpm up to add some additional fan cooling & coolant flow volume to check the temp rise.
I've not had a return of the runaway rpm since I replaced the IAC. My faulty IAC was the Factory OEM, so was another reason I chose the BWD brand, in case there was a OEM/Ford design problem, maybe I'd not get bit again. There haven't been many reports on this forum of this happening over the years, but in all cases I remember, replacing the IAC fixed it, so maybe you've put the fix on this puppy too!!!
More thoughts for consideration. Let us know how it goes over time.
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Well... it seems i spoke too soon 
the problem manifested itself again on the way home tonight 3500 rpm idle all of a sudden. i switched off the motor and restarted and everything went back to normal.
i'll do KOEO and read the codes out tomorrow.
other folks seem to have had this issue as well:
https://www.ranger-forums.com/4-0l-o...lution-124212/
the problem manifested itself again on the way home tonight 3500 rpm idle all of a sudden. i switched off the motor and restarted and everything went back to normal.i'll do KOEO and read the codes out tomorrow.
other folks seem to have had this issue as well:
https://www.ranger-forums.com/4-0l-o...lution-124212/
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Got around to inserting jumper and read out the codes (KOEO test).
i saw 111 then 111 then long gap then 1 then long gap then 122 and 122.
So according to:
Fuel Injection Technical Library » 3 Digit Self-test Codes 111-171
it looks like i have a system pass during the test, but an intermittent TPS signal during continuous test (while the engine is running on the road).
i'll replace that TPS and hopefully it fixes the issue. Gotta drain coolant and pull that coolant hose since i cannot get to those screws (they are very tight as well).
Sorry i am not used to messing around with the EEC-IV injection used in these Fords... more of a Bosch L-Jet and K-Jetronic guy.
EDIT:
UGH... why Ford why?!!!
TSB 89-14-07 TPS Screws
Publication Date: JULY 14, 1989
FORD: 1983-89 CROWN VICTORIA, MUSTANG, TEMPO, THUNDERBIRD
1984-89 ESCORT
1986-89 TAURUS
LINCOLN-MERCURY: 1983-89 CONTINENTAL, COUGAR, GRAND MARQUIS, MARK VII, TOPAZ, TOWN CAR
1984-87 LYNX
MERKUR: 1985-89 ALL MERKUR LINES
LIGHT TRUCK: 1983-89 ALL LIGHT TRUCK LINES
ISSUE: All Throttle Position Sensor (TPS) mounting screws have a "Pozidrive" head. The "Pozidrive" head looks a lot like a Phillips screw head. Use of a Phillips head screwdriver to remove a "Pozidrive" screw will normally result in a rounded or damaged screw head. Many technicians think it is an overtorqued or "frozen" screw. An adhesive was also used as a thread sealant starting with the 1988 model year. This adhesive requires still more effort to loosen and remove the screw.
ACTION: To prevent rounded or damaged screw heads when servicing the TPS mounting screws, technicians should always use a #2 "Pozidrive" screwdriver.
i saw 111 then 111 then long gap then 1 then long gap then 122 and 122.
So according to:
Fuel Injection Technical Library » 3 Digit Self-test Codes 111-171
it looks like i have a system pass during the test, but an intermittent TPS signal during continuous test (while the engine is running on the road).
i'll replace that TPS and hopefully it fixes the issue. Gotta drain coolant and pull that coolant hose since i cannot get to those screws (they are very tight as well).
Sorry i am not used to messing around with the EEC-IV injection used in these Fords... more of a Bosch L-Jet and K-Jetronic guy.
EDIT:
UGH... why Ford why?!!!
TSB 89-14-07 TPS Screws
Publication Date: JULY 14, 1989
FORD: 1983-89 CROWN VICTORIA, MUSTANG, TEMPO, THUNDERBIRD
1984-89 ESCORT
1986-89 TAURUS
LINCOLN-MERCURY: 1983-89 CONTINENTAL, COUGAR, GRAND MARQUIS, MARK VII, TOPAZ, TOWN CAR
1984-87 LYNX
MERKUR: 1985-89 ALL MERKUR LINES
LIGHT TRUCK: 1983-89 ALL LIGHT TRUCK LINES
ISSUE: All Throttle Position Sensor (TPS) mounting screws have a "Pozidrive" head. The "Pozidrive" head looks a lot like a Phillips screw head. Use of a Phillips head screwdriver to remove a "Pozidrive" screw will normally result in a rounded or damaged screw head. Many technicians think it is an overtorqued or "frozen" screw. An adhesive was also used as a thread sealant starting with the 1988 model year. This adhesive requires still more effort to loosen and remove the screw.
ACTION: To prevent rounded or damaged screw heads when servicing the TPS mounting screws, technicians should always use a #2 "Pozidrive" screwdriver.
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