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Bad stumble from 1800 rpms on up?!

mtrdrms

mtrdrms

1/2 ton status
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Mar 14, 2005
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Torrington, CT
My 87 was running well until just recently when there seemed to be a little hitch in the power at highway speeds. I would be cruising around 70 and it would feel like someone tapped my brakes?! Now it happens all the time and at 1800 RPMS. It really falls flat on its face. Total loss of power. I cannot climb a hill on the highway or even start from a stop without an issue. I am at a loss here. The truck is totally redone and the motor only has about 10k on it.
My first thought was that the fuel pump was going but that doesnt seem right because the drop is so predictable at 1800 rpms. I checked the plugs and it appears to be running lean so I was thinking that it is a fuel starvation issue. I also pulled the o2 sensor and cleaned it but it looked fine too. COuld it be something with the FI? I dont know but it is getting to be a real issue so any suggestions would be great.
 
Absolutely.

That will be your easiest troubleshooting check - go for it. Sounds very similar to when the filters go bad. And they are some of the most sensitive filters I have seen on any vehicle.
 
Well I changed the filter and from what I can tell from a short trip around the block, it totally fixed it. Thanks for the insight my friend.
 
I keep a spare fuel filter in my tool box cuz I can never count on a normal PM schedule with these things. And I have a new tank with less than 15k miles on it....

Glad you are at full power!
 
Well lets not get too excited. I took it to work today and unfortunatly the problem remains. I am wondering if it has something to do with timing? Maybe the timng advance (vacuum right?) is not working. It is the darndest thing because it is exactly at 1800rpms. I will be climbing a hill on the highway and the truck pulls hard right up till 1800 rpms then it feels like the motor just looses half power and it begins to slow right untill it gets below 1800 again then it pulls hard untill it hits 1800 and the cycle continues. I just dont get it.
The only thing I am thinking now aside from a computer problem (YIKES) is that the only original part of the engine is the distributor. How would I diagnose the distributor? Any help would be great and keep the good advice coming 2Dogs!
 
Ah! It's the 1800RPM dilithium crystal limiting switch......

DCLS - get it from Napa ;)












JK, I will think about this and give you a response. More info though - in reading your last 2 tests you said the evening trip right after replacing the filter was a success - are you sure? If so was the vehicle at normal operating temperature both times?
 
i am no expert but if you have the computer advanced timing and not vacume advance. i would say it is your pickup in the distributor. i just had a very simular problem and i slid in another distributor on my lunch break the other day and mine almost cleared up 100%. i still have other issues though i am running a 383 with factory tbi with no chip. but it cleared up my loss of power and stumble.
 
2Dogs said:
And is it 1800 RPM in all gears? Check that ASAP and report back pls.
Click to expand...

I checked the DLCS and had my neighborhood nuclear engineer test it and its good so I ruled that out a long time ago! OK the 1800 stumble IS in all gears. However like you said it was not nearly as evident when the engine was warming up before it reached operating temp. However I cant totally confirm this because I didnt really romp on it untill it got warm but as I said either way it is not nearly as evident in closed loop.
 
S. Burnham said:
i am no expert but if you have the computer advanced timing and not vacume advance. i would say it is your pickup in the distributor. i just had a very simular problem and i slid in another distributor on my lunch break the other day and mine almost cleared up 100%. i still have other issues though i am running a 383 with factory tbi with no chip. but it cleared up my loss of power and stumble.
Click to expand...
What distributor did you go with? Stock? Thanks for the input thats the direction I am leaning now too. I just want to hear back from 2dogs. Also check out TBIchips.com they will hook you up. I got my chip for less than a hundred bucks in less than a week. It cleared up my lean-running issues (like I suppose you are having with a 383).
 
I am far from the expert here - you should PM DYEAGER with a link to this post. There are a few other true Technicians on this site.

Considering that the stumble happens at 1800 RPM in all gears puts this one out of my league/experience.

To make this faster you should post a summary of all the symptoms:
* 1800 RPM issue
* all gears
* worse when warm (?)
* replaced fuel filter with no affect
* and any other fact or observation

I'll be here :o
 
You say 1800RPM's, is this idle AND under load (in any gear), or is it ONLY freeway? I assume only freeway.

If you floor it when it starts doing this, problem go away, or is like a brick wall no matter what you do when you hit 1800RPM?
 
This is only under load (good question) and not at an idle. If I sit in the driveway I can rev it and it doesnt stumble. It is not only freeway that this happens however. It will happen no matter where I am and in any gear uphill and downhill. When I floor it when it stumbles it doesnt get any better. It will accellerate, but it does so very slowly and it doesnt sound good when it does.
 
Following 2Dogs advice here is a list that might help.
Mildly built 350 TBI with Holley unit (about 5k on motor and setup).
Aftermarket TBI chip from TBIChips.com
SM465
New Fuel pump (1k)
Replaced fuel filter
Cleaned TBI
Reset IAC
Checked plugs and wires
 
Catalytic converter....?

If not it could be fuel pump, coil as mentioned above, plugs/wires/cap/rotor.

Thanks for joining Dorian!!
 
mtrdrms said:
Following 2Dogs advice here is a list that might help.
Mildly built 350 TBI with Holley unit (about 5k on motor and setup).
Aftermarket TBI chip from TBIChips.com
SM465
New Fuel pump (1k)
Replaced fuel filter
Cleaned TBI
Reset IAC
Checked plugs and wires
Click to expand...

How long after the new mods was it running well? Is it an auot or manual tranny?
 
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