Yet another 350 tbi problem

[GeorgiaK10]

My grandpa restored his 1987 4x4 350 tbi silverado for my 16th birthday and as his dying wish to see it in its orignial condition. It now has perfect black paint and really nice interior but since he passed away this truck has been problem after problem. When i fire it up it was idle smooth for about 5 seconds then idles really lumped and backfires through the exhaust and spits black smoke i have to keep my foot on the gas or else it sputters and dies after about 1 minute or 30 seconds of this it mellows out and idles perfect. This only happens when i start it. . Ive replaced the iac, both coolant sensors, and the egr. Is there some thing im missing? I recently had my ecu replaced bc some water got on it while i was driving. This started after my ecu went bad and the idle has progressivly got worse. Also my transmission keeps down shifting into 3 and only locks down some times? Do yall have any tips or know of anyone in the midlle georgia area that is good with tbi and can help me out?

 

[Honky Kong jr]

@1987 GMC Jimmy or @chengny should be able to help you.

 

[GeorgiaK10]

Any advice @1987 GMC Jimmy @chengny ?

 

[Honky Kong jr]

Any advice @1987 GMC Jimmy @chengny ?

Be patient man when they check in there will be more good info then you can imagine..

 

[chengny]

My grandpa restored his 1987 4x4 350 tbi silverado for my 16th birthday and as his dying wish to see it in its orignial condition. It now has perfect black paint and really nice interior but since he passed away this truck has been problem after problem. When i fire it up it was idle smooth for about 5 seconds then idles really lumped and backfires through the exhaust and spits black smoke i have to keep my foot on the gas or else it sputters and dies after about 1 minute or 30 seconds of this it mellows out and idles perfect. This only happens when i start it. . Ive replaced the iac, both coolant sensors, and the egr. Is there some thing im missing? I recently had my ecu replaced bc some water got on it while i was driving. This started after my ecu went bad and the idle has progressivly got worse. Also my transmission keeps down shifting into 3 and only locks down some times? Do yall have any tips or know of anyone in the midlle georgia area that is good with tbi and can help me out?

4L60 transmission?

If so you might want to check the P/N switch. That signal is one of the inputs the ECM uses for IAC valve control. It's also one of the inputs (via the VSS) involved in TCC operation.

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But whatever the reason, it sounds like the engine is running extremely rich at start up. Have you checked the TPS for smooth operation.

 

[GeorgiaK10]

4L60 transmission?

If so you might want to check the P/N switch. That signal is one of the inputs the ECM uses for IAC valve control. It's also one of the inputs (via the VSS) involved in TCC operation.

It has a 700r4. Would the P/N switch be the same?

 

[GeorgiaK10]

4L60 transmission?

If so you might want to check the P/N switch. That signal is one of the inputs the ECM uses for IAC valve control. It's also one of the inputs (via the VSS) involved in TCC operation.

You must be registered for see images attach

But whatever the reason, it sounds like the engine is running extremely rich at start up. Have you checked the TPS for smooth operation.

No i havent done anything with the tps. Was reading about it and it sounded like some people were having more trouble with it after tinkering so i was kinda nervous about it. How should i go about checking the tps

 

[87ChevyR10]

It has a 700r4. Would the P/N switch be the same?

No. The 700r4 is not a computer controlled tranny. It has a cable (TV cable) to auto shift gears.

 

[GeorgiaK10]

No. The 700r4 is not a computer controlled tranny. It has a cable (TV cable) to auto shift gears.

So my trans only slips out of 4th gear.. would ajusting the tv cable change all my shift points?

 

[87ChevyR10]

So my trans only slips out of 4th gear.. would ajusting the tv cable change all my shift points?

It might improve your shift points. What you need to do is push the TV cable all the in at the TBI. It has a tab you need to push in at the same time. Once all the way in, with engine off, go WOT on the TBI at the TBI. This will set the TV cable.

 

[GeorgiaK10]

It might improve your shift points. What you need to do is push the TV cable all the in at the TBI. It has a tab you need to push in at the same time. Once all the way in, with engine off, go WOT on the TBI at the TBI. This will set the TV cable.

So my transmission has slowly gotten worse and worse.. do you think it could just be that the cable has come loose and that is causing the downshift? Im gonna reset it tomorrow like you said. Thanks for the tip.

 

[1987 GMC Jimmy]

Has it given you a Service Engine Soon light when this happens or otherwise? I like the idea of the TPS, as well. If the idle voltage is way too high when you first start it up, it's just gonna dump fuel, which will cause black smoke and backfiring. I'd like to see the step progression of your IAC valve and your TPS voltages at idle, WOT, and everything in between. Here's how to make a scan tool that utilizes a cheap, homemade cable and public domain software for your laptop.

http://www.gmsquarebody.com/threads/connecting-with-a-laptop-with-tuner-pro-rt-and-diy-cable.13194/

Here are the tests for the TPS sensor. This is intended for later model vehicles, but it's the same setup as the older ones.

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/tp-sensor-tests-1

Was your ECM replacement a reman or a used unit? Is it giving you an SES light when you first turn the key on?

Are you sure you're having lockup issues, or is this a staying in fourth gear issue? If it is a lockup issue, it could be caused by a number of things such as P/N switch (as mentioned), VSS buffer, brake switch, TCC solenoid, fourth gear pressure switch, the torque converter clutch solenoid itself or an ECM problem. If the shift points are what's giving you the trouble, the TV cable should be the first thing on your list, as mentioned. Make sure it's not frayed, either. That cable clips on the carb/TBI pretty well. I doubt it'd just fall off. Now, if someone left it disconnected, that's a totally different and potentially very bad story. If it's misadjusted, that's not a big deal as long as it wasn't that way for too long.

 

[GeorgiaK10]

Has it given you a Service Engine Soon light when this happens or otherwise? I like the idea of the TPS, as well. If the idle voltage is way too high when you first start it up, it's just gonna dump fuel, which will cause black smoke and backfiring. I'd like to see the step progression of your IAC valve and your TPS voltages at idle, WOT, and everything in between. Here's how to make a scan tool that utilizes a cheap, homemade cable and public domain software for your laptop.

http://www.gmsquarebody.com/threads/connecting-with-a-laptop-with-tuner-pro-rt-and-diy-cable.13194/

Here are the tests for the TPS sensor. This is intended for later model vehicles, but it's the same setup as the older ones.

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/tp-sensor-tests-1

Was your ECM replacement a reman or a used unit? Is it giving you an SES light when you first turn the key on?

Are you sure you're having lockup issues, or is this a staying in fourth gear issue? If it is a lockup issue, it could be caused by a number of things such as P/N switch (as mentioned), VSS buffer, brake switch, TCC solenoid, fourth gear pressure switch, the torque converter clutch solenoid itself or an ECM problem. If the shift points are what's giving you the trouble, the TV cable should be the first thing on your list, as mentioned. Make sure it's not frayed, either. That cable clips on the carb/TBI pretty well. I doubt it'd just fall off. Now, if someone left it disconnected, that's a totally different and potentially very bad story. If it's misadjusted, that's not a big deal as long as it wasn't that way for too long.

Im pretty sure it is a lock up issue bc i installed a tach and i can see it locking in 4th then it just comes right back out. No check engine light during rough idle. But check engine comes on when i first put key in like it should. Both of these problem came after my old mechanic put in the ecm. Im gonna do some reading about making that scan tool.

 

[87ChevyR10]

Im pretty sure it is a lock up issue bc i installed a tach and i can see it locking in 4th then it just comes right back out. No check engine light during rough idle. But check engine comes on when i first put key in like it should. Both of these problem came after my old mechanic put in the ecm. Im gonna do some reading about making that scan tool.

You could go to AutoZone and have them scan it for free.

 

[1987 GMC Jimmy]

Do Autozones test OBD I? The ones I've been to only fool with OBD II. The tach was a good diagnostic tool for that. Hmm... The mechanic would be an easy scapegoat, but there are only a couple things that I can think of that would have went wrong there. The ECM that went in there could have been bad even though your diagnostic circuit works as it should, which I wouldn't put it past either a junkyard or reman unit. ECMs share connectors, too, so it is possible, however unlikely, that the wrong ECM is in there. If it's a reman unit, it won't have the ID sticker on it, but if it's used, it should be a 1227747 ECM. I know the 1227165 ECM shares the connector set, and those come out of 86-89 F Bodies. Again, I don't think this is likely. Those are TPI cars that use a MAF system. I don't even think yours would run in that situation, but I haven't tested this so I have no clue what exactly it would do. It's fishy that it starts acting up after the ECM swap, though. It'd make sense that it's related to that, but it's also possible that they have nothing to do with it. Pin F on the ALDL is for TCC diagnostics. I know you can probe it to test the brake switch (12V resting, 0V with brake depressed), and I've read of people on here making manual switches that simply bridge Pin A (ground) and Pin F together, but the info is pretty vague. Maybe Jerry knows, and I'd like a more concrete explanation on the matter myself. @chengny