Engine Backfires and idles rough
Thread Starter
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Junior User
Joined: Jan 2019
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Engine Backfires and idles rough
Hi, my 1993 f250 5.0L has an issue with backfiring. If the truck is warmed up and used then parked for 5-20minutes then restarted, it will backfire for about 5 more minutes after. I've looked on online forums and have not found any explanation for this issue. The motor once running for a bit will have a unsteady idle to the motor almost shutting off in a few instances, almost sounds like its coughing. I looked on the forums and did as other people sugested but no success. I've checked vaccum lines, replaced the IAC valve, the TPS sensor, The entire throttle body, and the MAP senor. no success.
Cargo Master
Joined: Jun 2013
Posts: 3,182
Likes: 15
From: Somewhere n North America
Check for Codes, run the KOEO & KOER Self-Test, let your computer tell you what it knows;
Engines are easier to diagnose over the internet when all the codes are listed, and in order of test.
KOEO - (O) Self-Test Codes =
KOEO - (C)(CM) Continuous Memory Codes =
KOER - (R) Self-Test Codes =
Check Base Timing?
Check Timing Advance?
Do you have any Non-Stock modifications?
(Air Intake? Smog System? Exhaust System? Ignition System?)
Engines are easier to diagnose over the internet when all the codes are listed, and in order of test.
KOEO - (O) Self-Test Codes =
KOEO - (C)(CM) Continuous Memory Codes =
KOER - (R) Self-Test Codes =
Check Base Timing?
Check Timing Advance?
Do you have any Non-Stock modifications?
(Air Intake? Smog System? Exhaust System? Ignition System?)
Last edited by vjsimone; Feb 11, 2019 at 04:25 PM. Reason: Adding Text
Thread Starter
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Junior User
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I ran the self test both while the engine was running and when off but on the "On" position, codes listed bellow. There are non stock modifications to the truck. I had high idle on the truck so I replaced the Throttle body with a twin 56mm BBK one, as well as the TPS sensor and IAC valve were changed. The high idle was corrected although the surging idle was more pronounced after that. The exhaust was replaced from factory to a true dual set up with longtube headers, the O2 sensor was installed onto only one of the dual exhaust pipes as close to the headers as possible. I would also like to add that the EGR was never reconnect to the exhaust. It might seem like the modifications are the cause however this issue started long before I made these modifications which makes me believe there is another issue.
With the engine off I got two codes:
23: Throttle position sensor out of self test range
31: EGR position sensor bellow minimum voltage.
With the engine running I got three codes:
23: Throttle position sensor out of self test range
42: Oxygen sensor circuit indicates system always rich
31: EGR position sensor bellow minimum voltage.
With the engine off I got two codes:
23: Throttle position sensor out of self test range
31: EGR position sensor bellow minimum voltage.
With the engine running I got three codes:
23: Throttle position sensor out of self test range
42: Oxygen sensor circuit indicates system always rich
31: EGR position sensor bellow minimum voltage.
Cargo Master
Joined: Jun 2013
Posts: 3,182
Likes: 15
From: Somewhere n North America
Ok, so make some measurements of the TPS, Ref Voltage should be 5v, Signal output to Computer at closed throttle should be around 1 volt, measure Wide Open Throttle and verify the ground wire.
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Junior User
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on the BBK throttle body I there was 3 vacuum ports If i counted correctly. One is hooked up to the fuel vaporisation canister If i'm not mistaken, The other is capped off, I didnt have any vac lines to hook up to it so I used a cap that came in the box.. The last vac connection dint have a cap nor a line to connect so that is left open. I did block it to see if that was my issue and there was no change.
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Freshman User
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From: Oregon
Backfire and unsteady idle... Check base timing yet? Does it run *great* before your hot restart issue?
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Cargo Master
Joined: Jun 2013
Posts: 3,182
Likes: 15
From: Somewhere n North America
And you will want to also check to see if the timing advances, both when the engine is cold and when the engine is warm, and during the Backfiring.
Cargo Master
Joined: Jun 2013
Posts: 3,182
Likes: 15
From: Somewhere n North America
Grey/White stripe wire is Signal to PCM.
Brown/White stripe is Ground wire.
Hotshot
Joined: Jun 2015
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There are two vacuum ports on the driver’s side for earlier style throttle bodies and one on the passenger side for the later style. Are you saying you left a port uncapped or connected to a vacuum hose?
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Junior User
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The passenger side was hooked up, The upper (bigger) vacuum connection was capped, the lower (smaller) vacuum line was not capped or hooked up, it is left open. The BBK kit did not come with a cap for that connection and the truck did not have a line to connect to it. I'm unsure what the purpose of them are, although I have blocked it off with my finger for a while and it did not change how it ran.
Timing needs to be checked as well, and I have not check the TPS voltage yet.
Timing needs to be checked as well, and I have not check the TPS voltage yet.
Cargo Master
Joined: Jun 2013
Posts: 3,182
Likes: 15
From: Somewhere n North America
The passenger side was hooked up, The upper (bigger) vacuum connection was capped, the lower (smaller) vacuum line was not capped or hooked up, it is left open. The BBK kit did not come with a cap for that connection and the truck did not have a line to connect to it. I'm unsure what the purpose of them are, although I have blocked it off with my finger for a while and it did not change how it ran.
Hotshot
Joined: Jun 2015
Posts: 12,870
Likes: 2,443
The passenger side was hooked up, The upper (bigger) vacuum connection was capped, the lower (smaller) vacuum line was not capped or hooked up, it is left open. The BBK kit did not come with a cap for that connection and the truck did not have a line to connect to it. I'm unsure what the purpose of them are, although I have blocked it off with my finger for a while and it did not change how it ran.
Vacuum connection on my ‘87
You cannot leave an open vacuum port. My hardware kit came with three vacuum caps.
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Junior User
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I capped it, there was no change in how it ran. The truck only will backfire for the first couple minutes after a hot start. I also checked my timing. It was at 20BTC, factory is 10 I believe, I lowered it to about 15BTC but iradic Idle is still present. I noticed that my timing light seemed to skip a flash about when the RPM would drop, I'm not sure if my timing light has an issue, or possibly the truck is misfiring although it does not sound that way. I would like to note that the distributor cap and rotor was replaced a year ago, along with the ignition coil, spark plugs and spark plug wires so I believe a misfire is unlikely.