1st TBI Conversion Help Needed

[MtBraun]

I’m not familiar with the TPS relearn procedure. I’m really upset in how this has worked out. Not because of you in any way, Matt, but it’s usually not that stubborn. People have had chronic TBI issues on here a few times before, but the threads usually die after a few weeks so we don’t see the resolution. Sometimes there is, and I can easily assume that sometimes there isn’t. It sucks to invest a bunch of time and money in something and have it flop. I thought of one thing here. Have you tried backprobing the entire TBI harness at the ECM to make sure all your voltages leaving and coming back there are to spec? I really want to nail this son of a bitch because you’ve been through too much to return to the old status quo. Or at least that’s how I feel.

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I’d go through and probe everything both in and outside of this chart that I could. After that, you’re confined with to more mechanical/conventional solution. That’s how I see it. Have you tried to run it with the MAP sensor port plugged off at the TBI? I may experiment with that, too.

I did this on the old hacked up junkyard harness, but not on the new AFI harness. That's a good thought. Since I had the timing set and I didn't observe the guy setting it, I'm gonna pick up a test light and double check timing. for as much as it backfires through the throttle body it suggests a potential timing issue. I'm also going to troubleshoot the entire electrical system from the dizzy to coil. The only two sensors not replaced are the ignition module and the MAP sensor. I did not consider running it without the MAP sensor plugged in. I presume this will throw a code? What do I look for to determine if the MAP sensor is doing its job?

I'm pretty persistent with these types of things and not giving up quite yet. I was just pretty frustrated this morning. Putting on the "dad" hat, there is a lesson here for my son... be tenacious and don't give up.

 

[4WDKC]

I did this on the old hacked up junkyard harness, but not on the new AFI harness. That's a good thought. Since I had the timing set and I didn't observe the guy setting it, I'm gonna pick up a test light and double check timing. for as much as it backfires through the throttle body it suggests a potential timing issue. I'm also going to troubleshoot the entire electrical system from the dizzy to coil. The only two sensors not replaced are the ignition module and the MAP sensor. I did not consider running it without the MAP sensor plugged in. I presume this will throw a code? What do I look for to determine if the MAP sensor is doing its job?

I'm pretty persistent with these types of things and not giving up quite yet. I was just pretty frustrated this morning. Putting on the "dad" hat, there is a lesson here for my son... be tenacious and don't give up.

When you did the swap did you use the TBI balancer from that truck?

 

[MtBraun]

When you did the swap did you use the TBI balancer from that truck?

No - should have I?

 

[1987 GMC Jimmy]

That’s interesting. I checked the PNs for aftermarket balancers on a 1991 and 1978 350, and they were different. I checked the specs and all were the same except the ring width. I’d feel fishy about reusing an old balancer from a different engine or even the same one if it was old. The rubber warps, gets brittle, and cracks over time. Sometimes less time than you’d think. I’ve also seen in dry rot into chunks.

 

[MtBraun]

I reset the IAC for like the 10th time, fired the truck up and it idles ok...the symptoms now are kind of back to where they were. It doesn't fire right up, kind of have to goose it a bit. But it is driveable.

I checked timing with my flashy new timing light and it was perfect at 0 degrees with engine warm and bypass unplugged. I also used that flashy new tool to check for spark, and I have spark on every plug wire when cranking over. Based on some other reading, this tells me the dizzy, ignition coil, pickup coil and ignition module are functioning as required. I guess it doesn't mean I have quality spark, but I do have spark nonetheless.

I checked idle with timing light and it was about 500 RPM. So I did the IAC reset/idle set procedure according to the earlier referenced gearhead post. I set idle about 600 rpm with IAC unplugged. When I started truck back up the idle settled around 600. I thought it should be closer to 700-750 which is what the BIN should call for. In this post Mark says target IAC should be 10-20 when warmed up. My IAC starts high and goes all the way down to zero. I did this three times to confirm and it does exactly the same thing each time.

I think now it may be down to a tune change needed. I am sending a spare ECM off to get the ZIF socket installed and a custom tune based on some logs I need to create yet.

...stay "tuned"

 

[MtBraun]

That’s interesting. I checked the PNs for aftermarket balancers on a 1991 and 1978 350, and they were different. I checked the specs and all were the same except the ring width. I’d feel fishy about reusing an old balancer from a different engine or even the same one if it was old. The rubber warps, gets brittle, and cracks over time. Sometimes less time than you’d think. I’ve also seen in dry rot into chunks.

Could be that despite being basically the same they re-numbered the part. This happens often with change in models, etc. I don't think I'm going to change anything there.

 

[4WDKC]

That’s interesting. I checked the PNs for aftermarket balancers on a 1991 and 1978 350, and they were different. I checked the specs and all were the same except the ring width. I’d feel fishy about reusing an old balancer from a different engine or even the same one if it was old. The rubber warps, gets brittle, and cracks over time. Sometimes less time than you’d think. I’ve also seen in dry rot into chunks.

If I remember correctly the older engines get timed at the 12 oclock position for 0 where as my tbi has a timing mark 0 is about the 2 o clock position. So while it could be physically the same, the timing mark could be in a completely different spot. Your tdc in the 12 position could be like 50* TDC if the wrong balancer/timing mark combination is used.

 

[1987 GMC Jimmy]

If I remember correctly the older engines get timed at the 12 oclock position for 0 where as my tbi has a timing mark 0 is about the 2 o clock position. So while it could be physically the same, the timing mark could be in a completely different spot. Your tdc in the 12 position could be like 50* TDC if the wrong balancer/timing mark combination is used.

I also saw that when I was researching it. I guess it’s not the end of the world since the whole timing set is from the original motor.

 

[Dougnsalem]

Have you tried to run it with the MAP sensor port plugged off at the TBI? I may experiment with that, too.

X2 on the MAP sensor. Would be nice to get a data stream and see what it's doing. I've been burned with the mismatched balancer/timing cover. The correct matching pair can go on either motor, so a agree with you that he should be fine

 

[1987 GMC Jimmy]

X2 on the MAP sensor. Would be nice to get a data stream and see what it's doing. I've been burned with the mismatched balancer/timing cover. The correct matching pair can go on either motor, so a agree with you that he should be fine

There are some screenshots from TunerPro showing MAP sensor data on previous pages. It’s hard to tell with only a few frames in time, though. The wonky vacuum stuff made me think of that because they can do weird stuff sometimes. I think his flatlining IACV is a biggie. Maybe and hopefully after that’s addressed, it’ll work better or correctly.

 

[Dougnsalem]

There are some screenshots from TunerPro showing MAP sensor data on previous pages. It’s hard to tell with only a few frames in time, though. The wonky vacuum stuff made me think of that because they can do weird stuff sometimes. I think his flatlining IACV is a biggie. Maybe and hopefully after that’s addressed, it’ll work better or correctly.

Ok. I missed that earlier. Seems like you're on the right track with the map/iac stuff. At least it makes sense....


There were different versions for cars and trucks depending on year and application. Its best to verify that balancer and timing tab match at TDC on #1:

Again, not an issue here....

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[MtBraun]

Ok. I missed that earlier. Seems like you're on the right track with the map/iac stuff. At least it makes sense....


And just to clarify for the future. 4WDKC was right about the different timing locations, but the newer one is at 12 o'clock

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Again, not an issue here....

Hmmmm, this is interesting. My TDC is at 12 o'clock.

 

[Dougnsalem]

Hmmmm, this is interesting. My TDC is at 12 o'clock.

Your balancer timing mark(the slot) is at 12, with timing gears indexed(both up)? Do you have the old tab location? But that balancer was already in use with that tab, right?

Edit: SO, did you have to put in a TON of advance to get it to run right, with the carb setup?

 

[MtBraun]

Your balancer timing mark(the slot) is at 12, with timing gears indexed (pointing to each other)? TDC for #6. Do you have the old tab location? But that balancer was already in use with that tab, right?

The balancer timing mark is at 0 on the tab at 12 o'clock. I don't understand your second question (TDC for #6.)

I don't recall another tab location. The power steering pump is in the way, but I will look.

That tells me it isn't the original engine cover and balancer combo.

But it shouldn't matter as long as the two are paired correctly, right?

I have never removed the balancer or timing cover, and the truck ran great carbed before I began this debacle.

 

[MtBraun]

There are some screenshots from TunerPro showing MAP sensor data on previous pages. It’s hard to tell with only a few frames in time, though. The wonky vacuum stuff made me think of that because they can do weird stuff sometimes. I think his flatlining IACV is a biggie. Maybe and hopefully after that’s addressed, it’ll work better or correctly.

After you pointed out the vacuum issue I paid closer attention to it when driving, and I think I just caught a frame precisely when it backfired/cutout, which dramatically lowered the vacuum. When driving it since it holds a pretty steady 18-19 vacuum. It will dip when accelerating, but then recovers to the correct level.

I found this though on IAC counts at zero, meaning it is completely closed.

There is a vacuum leak allowing unmetered air to enter the engine

· The throttle stop screw is adjusted incorrectly (throttle being held open too far; more on this later)

· There is a problem with the throttle cable or cruise control system that isn’t allowing the throttle to close all of the way

· The IAC valve itself is faulty

Soooo, the only vacuum related item I haven't replaced is the MAP and the check valve at the brake booster. I never sprayed the brake booster with ether either. Never thought of it till now. I'm confident the TB base gasket and other vacuum lines into the TB are sealed.

To test the MAP, simply unplug it and plug the TB port? If it runs better then that is the problem?