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TBI idle surge and stumble: UPDATED SOLVED

C

Cricket

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TBI is surging with steady part throttle in open loop when cold. Especially when it's been driven - shut down for an hour - and restarted. The surge is like a whup-whup-whup-whup until I add throttle and rev it out. It goes away after I drive it for a minute or it warms up to closed loop.

It does this with both old and new TPS but with the new TPS the acceleration stumble remains. All other sensors are new.

Any ideas? :dunno: The open loop/ closed loop is throwing me.
 
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Is this a completely stock setup? If not, can you list the mods? Did the problem first occur after any work? Has the check engine light ever come on?

KM
 
Stock set-up off a 1990 C1500, painless harness, VSS, Gerotor pump on a stock saddle tank, all new sensors.
 
Have you driven it at a constant 30mph-ish just to let the ecm relearn the parameters?
Have you connected the N/P sensor? If the ECM doesn't know you're in neutral or drive it can cause that effect.
Having said all that, my '05 Ram does it occasionally!
 
WHere excactly is the N/P sensor located, Mark? I'm having a few "stumbling" issues myself at idle, but only in Park. Once the sucker is thrown into Drive, the idle smoothes right out.:confused:
 
CanmoreK5 said:
WHere excactly is the N/P sensor located, Mark? I'm having a few "stumbling" issues myself at idle, but only in Park. Once the sucker is thrown into Drive, the idle smoothes right out.:confused:
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On the base of the column shift. It acts as an inhibitor and a switch to tell the ecm that you're in neutral or park so it can compensate for higher idle without load. When I put a B&M shifter in my '78 I forgot to connect it up and it acted a bit screwy.
My '91 has never had a 'really' steady idle, but checking it with Winaldl it seemed that it was just a result of it continually trying to lean it out then bring it back again which is apparently correct. I think my ecm was just a bit too enthusiastic!
 
Does it rev out smoothly in neutral when you're having trouble? Is it like a flooding problem?
 
Neutral or drive, any super light part throttle application would create a series of jerks or surges as if you were purposely tapping the throttle in a steady drum beat. If you just popped a little deeper into the throttle it would disappear.

The latest report: Okay I have had some positive results with fuel injector cleaner. I've run two tanks through with BBK and the surge has seemingly been eliminated. The throttle is much smoother and more responsive. My guess at this stage is that my injectors were a bit plugged up.

If the EGR had been sticking it would have thrown a code 32 - something I am all too familiar with.
 
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UPDATE: I found a very small vacuum leak at the EGR base gasket. Replaced the gasket and that got rid of the last bit of hesitation. The leak was small enough that it would not stumble/surge under regular acceleration but with powerbraking or feather light throttle it became very apparent.

I had two problems at once. Partially clogged injectors and a small vacuum leak. Now that the leak is sealed and the injectors are clean it's running great.

At least for today.:D Hallelujah.
 
Im working on a 94 with a check engine light and a knock sensor circuit code i replaced it and repaired the bad wire and connector. I did tune up timing etc. the problem has been there since i bought it . also the scan tool indicates that the truck is in d2. i no longer have a 700r4 and dont remember seeing a switch on the tranny.
 
my idle

it seems to surge alot, especially when in drive? any ideas?
 
Does it surge while accelerating or just idling in drive with the brake on. Warmed up or cold? Stock TBI or swap?
 
jiminycricket said:
Does it surge while accelerating or just idling in drive with the brake on. Warmed up or cold? Stock TBI or swap?
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brake on, both, and stock
 
Sounds like a fuel delivery problem. Take off your air cleaner and watch your injectors for a good spray pattern. Are they dripping at all? If you don't see a problem there then put on an inline fuel pressure gauge and check the pressure. Based on previous runability problems I've seen on the board it has usually been the in-tank fuel pump.
 
Here is another easy check with only two possible outcomes. Does the engine surge while holding the accelerator steady?

If so you have an intake leak or fuel delivery problem.
 
jiminycricket said:
Here is another easy check with only two possible outcomes. Does the engine surge while holding the accelerator steady?

If so you have an intake leak or fuel delivery problem.
Click to expand...

no it doesnt
 
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