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DAGGUM COMPUDERS drivability diag

obijuank5

obijuank5

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So at what point do I condemn a throttle body assembly? Cause I'm about ready to.
1990 5.7 stock af.

I do understand that this is ancient technology but it should be running better than this imo. Current symptoms are that when its warming up on a cold start it will stumble and like misfire a little and then not happen again when warm. At all times now it seems it cant hold any steady rpm, but used to only be noticeable under coast or no load but trying to keep steady speed. Leaning on this being another TPS that only lasted a few months. Standard motor products brand.
If I'm gonna shotgun any more money into this I might as well get throttle shafts and FPR stuff with it.
There is this on ebay which has me interested. TBI assembly

I have personally replaced:

tune up: plugs wires cap rotor and ignition module.
throttle position sensor
coolant temp sensor
idle air control
manifold pressure
fuel filter

unknown miles on everything else.
 
Stuff like this is so hard now days because you cannot trust any new parts to be 100% good. Plus add in being OBD 1 and it's even harder to t/s. Plus there's not a lot of great mechanics with this stuff around anymore. I have to learn everything as I go on my kid's '93 F350 460 gasser. My only advice is to start with the basics and check them first. Fuel pressure, battery volts, then go to checking sensor readings. Gonna need to get good with a voltmeter if you aren't already and if you don't want to just throw parts at it. If you can find someone with a scan tool that can hook to it even better. I have a really nice Snap-On one at work and bought the OBD 1 adapters to hook up to my kid's truck. Better than checking every sensor with a meter.
 
The whole distributor is suspect, I replaced the whole thing quite a bit, back in the day. We used to buy whole GM distributors from GM performance. Dealer only has separate parts.

ECT is can give some wild drivablity issues, be sure the connection (pins) are tight, and the ground is good.
 
If it only does it cold, I would first suspect a fuel delivery issue. That issue is *probably* being dealt with by the O2 sensor once it warms up with. Or at least as much as it can, which is why that specific problem disappears when it gets hot, then transitions to not running properly. Of course electrical stuff can act weird cold vs hot too. Fuel pressure test is basically free and you could be seeing low pressure symptoms.

Injectors are suspect at this age too, either worn out or clogged/dirty. I'm not sure what we can buy for cleaning injectors (cans of stuff that gets forced through the injectors once removed) is as good as what an injector shop can do, along with their experience identifying other problems with injectors, but it certainly can't hurt if they are likely original and have never been cleaned. You can find resistance values online and test yours hot, but that's not going to tell you anything other than the electrical condition of the injector.
 
Gonna echo the distributor. They are junk, and tear themselves up, giving you all kinds of weird issues. Pull the cap, have a look.
 
tRustyK5 said:
Gonna echo the distributor. They are junk, and tear themselves up, giving you all kinds of weird issues. Pull the cap, have a look.
Click to expand...
What should I be looking for? I recently did a cap and rotor but didn’t see anything out of the ordinary from an uneducated perspective. Are we talking like degree rotation differences from cam to rotor or something? I definitely didn’t check that.
 
I have seen distributors, specifically TBI versions, under the rotor area they chew themselves up. lots of rusty dust was the big visual clue. I had it happen on my 90 Blazer, and on a friend's 89 Blazer. Both started running weird, stumbling and missing and all the regular stuff didn't fix the problem. At the time I had a good stash of TBI era sbc parts, and swapping in a known good distributor fixed the issue.

At the least it's worth a more critical look. It might not be your issue, but it is a somewhat common failure point. The TBI distributors are nowhere near as robust and reliable as the OG HEI stuff was.
 
I appreciate that. So far my tbi education is a little all over the place and some more focused reading wouldn’t hurt. I appreciate the guidance.
 
One more thing you might want to check is the EGR. Might be a good idea to inspect the vacuum lines, clean everything up, new gasket and new EGR valve. They cause some similar drivability issues to what you're describing when they go bad.
 
yeah they like to stick open. Usual symptom, when slowing to a stop from above 40 mph they stall, or run very rough. Will start right up and run good till you get over 40 and the ecm commands the egr open. Sometimes this is a very dirty egr solenoid vent port filter, other time is carbon in the egr pintle area. If you think you might have a vacuum leak but aren't finding one, strike the egr with a hammer handle at idle, if engine smooths up after that it was leaking.

I always tied to re-torque the Intake manifold, after a few years I could always get 1/2 turn or so from the manifold bolts.
 
@tRustyK5 post got me thinking. All the distributors I ever replaced had rusty dust, as well, thou distributors I merely replace cap and rotor were had rust dust too.
I never had time to really figure out why these distributors failed. Right now I thinking that rust dust might be interfering with the pick up coil. Got me wondering if through cleaning would help.
 
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Used to have a 1994 that would require a new distributor every 4-5 yrs ,had the truck for 20+ years ,sold it with 130k on the odometer.
The day it sold ,buyer driving away,it died couple blocks from my place , trouble shooting , throwing parts at it , nothing. Dropped in a new distributor from Vatozone ,fired right up and away he went ,happy as can be.
 
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