Troubleshooting a running issue, TBI system.

American Delco GM ECUs and PCMs, ALDL, OBD 1.5.
ashhhhh
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Troubleshooting a running issue, TBI system.

Post by ashhhhh »

Hey guys,

Is it OK to post a "help me troubleshoot" type topic?
Ive got a TBi (700 series) which has an annoying drivability issue. I feel ive ruled out mechanical causes and im now bashing my head of the EFI system.
Ill post the details if anyone is interested to hear the tale of woe.

Cheers
Ash
pman92
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Re: Troubleshooting a running issue, TBI system.

Post by pman92 »

Post up the story and I'm happy to help if I can
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ashhhhh
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Re: Troubleshooting a running issue, TBI system.

Post by ashhhhh »

Thanks mate. :thumbup:
So this is a Lada Niva, erase your bias at that point and understand that its a 1700cc 4 cyl which runs a Rochester 700 series TBI (Central, single injector) and a GM ECU. :D
Ive attached the wiring diagram so you can see which sensors it runs.

The issue is basically that it holds revs. When driving it will hold revs annoyingly high (2000-ish) which means you have to wait for them to drop before shifting gear.
When cold it is worse, coming to a stop will see it revving around 2000rpm and then sloooowly dropping to normal idle.
When revving the engine in neutral it does not drop in a linear fashion either, it hangs at around 1400rpm briefly or kind of pops back up after going lower.
I know some ECU's are programmed with this "rev hang", this is not one of them, other owners dont have the issue.

Ive done countless tests and checks so far, too many to type. Keen to hear your thoughts and ill tell you if ive tested in that area.
Ill start the ball rolling by saying ive ruled out VSS, which was my first thought. It new (not a guarantee I know) and the ECU reports correct speed.

Cheers!
Ash
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Gareth
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Re: Troubleshooting a running issue, TBI system.

Post by Gareth »

Has the 'idle speed adjuster' been checked for proper operation? Vacume leaks? TPS been checked? Is the throttle plate clean and free acting?

Please list all the checks you have done
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pman92
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Re: Troubleshooting a running issue, TBI system.

Post by pman92 »

I would be checking for vacuum leaks to start with. And be absolutely sure there is none. Including PCV leak, faulty brake booster etc etc.

If there is a throttle stop screw, make sure it is adjusted correctly. No one has wound the throttle open slightly to try and increase idle speed etc.

Make sure throttle cable isn't too tight holding throttle open very slightly.

Make sure throttle body is clean and IAC is clean.

If the TPS is adjustable make sure it it's adjusted correctly (I don't think it will be adjustable).
What is TPS signal voltage at closed throttle?

Sounds like the IAC stepper motor has to move a long way to get the correct idle speed due to too much air (vacuum leak or open throttle) hence the delay. OR TPS signal is saying open throttle, which means idle speed will not be controlled (hence high speed).

Or maybe the stepper motor itself is faulty. Looking at the diagram it's possibly the same stepper motor used on many other vehicles and shouldn't be hard to come by.
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ashhhhh
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Re: Troubleshooting a running issue, TBI system.

Post by ashhhhh »

My test to rule out vaccum leaks (aside from obvious manual checks with propane) was the extend the IAC pintle (it does this in "field mode") and pull the plug.
The car then idled well and at a low 600rpm idle. This seems normal to me. The idle speed screw is still capped so I know if hasnt been touched.
My logic here was to cut the idle control out of the equation and the vehicle drove great. (except for wanting to stall due to low idle of course)

Throttle and cable do not stick.

TPS not adjustable and new, EFI Live shows 0-100% response, looks good. (I dont recall the voltage but it was normal per the GM chart)

IAC I dont not think is faulty, per the above test. I fitted a spare and its exactly the same. These things are meant to be auto-resetting.
Ive cleaned the IAC passages and replaced the o-ring.

Injector pattern looks good, spare injector made no difference. New o-rings of course.

Backprobed the ECU pins and confirmed all stats are correct per GM charts.

Tried different coil pack, leads, plugs.

New Crank sensor.

It goes on and on. I guess this is really a trial and error thing, id like to rule out 100% what it will NOT be and then focus on whats left.

Ive got recording and raw data (EFI Live) is it helps. It didnt help me but I dont know what is "normal"
ashhhhh
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Re: Troubleshooting a running issue, TBI system.

Post by ashhhhh »

I think youre right above the IAC pintle being way out, I just dont know why it is doing that.
I have not done much with the EGR yet but nothing seems amiss at first look.
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Gareth
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Re: Troubleshooting a running issue, TBI system.

Post by Gareth »

If plugging the IAC port makes the idle not hang but it still wants to stall when driving then I back it in that it has a serious vac leak, probably the brake booster.

Best/easiest way to find vacume leaks is to fill the intake system with smoke under light pressure.

Can you see IAC steps in your data?
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ashhhhh
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Re: Troubleshooting a running issue, TBI system.

Post by ashhhhh »

I disagree, how would the idle be "factory stock minimum" correct if there was a vac leak? (genuine question, not being a dick)
Do you mean it only leaks under driving conditions?
Ive plugged the booster port and nothing changed.

Yep at a cool idle steps are mid 30's.
Warm idle its around 14.
Driving when warm I saw around 50 max.
Driving when cold I saw mid-60's to a max of 75 or so.
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Gareth
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Re: Troubleshooting a running issue, TBI system.

Post by Gareth »

Plugging the IAC should make the engine stall
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