73-87 Chevy _ GMC Trucks > Fuel Systems and Drivability

89 TBI 350 Rough Surging Idle, Cutting Out/Stumbling on Acceleration

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Dr_Snooz:
Hi everyone. It's been a while, but I'm back with my 89 Suburban. I'm dealing with an intermittent stumbling/cutting-out issue lately. This morning the truck drove great with only a few light stumbles, on light acceleration, as I went to church. Coming home, it had a rough, surging idle on startup, and a lot of stumbles all the way home. Sometimes the idle would dip low briefly, then recover as I came to a stop. By "stumbles," I mean momentary losses of power for a second or two before recovering. It hasn't died yet, but I'm sure that's coming.

I've been through a few troubleshooting charts listed in the factory manual without finding anything wrong. The charts don't inspire confidence, though. For example, if you hit 9-13 psi within 2 seconds of turning the key on, then the fuel pump is deemed good and you can look elsewhere for your problem. ?

I've replaced the distributor with a new NAPA unit. The wires are a couple months old. O2 sensor is a new Bosch. Plugs were inspected and gapped 3 months ago. Fuel pressure tested at 12 psi per above. Fuel pump was replaced with an AC Delco unit in 2015, and has very few miles on it. Injectors are new rebuilt units from Standard Motor. Alternator output is 14.7V at idle. Timing is 0 with the brown wire unplugged. TPS is new. EGR tests out okay per the manual. Spark at idle is strong (hard to check it at speed.) Fuel filter is new. There seems to be a small vac leak at the booster check valve. I'll fix it tomorrow. No ECM codes. Battery cables are new homemade jobs that greatly improved my starter speed.

I can't find any threads or weblinks that cover my specific issue. I'm open to suggestions.

Thanks

JohnnyPopper:
Sorry, your truck us too new for this forum...




























Glad you have a sense of humor.

With all that good stuff going for the TBI components, I would look for a ground issue. They are the nemesis most overlooked.

There have been a few good posts that actually 'red arrow' locations that many are not aware of. Myself included!

I'm struggling getting my '88 454 to behave. Tune up is next in line.

bd:
I hope you didn't replace all of those parts chasing the "stumble."

By all means, correct the vacuum leak.

Unplug the EGR valve to see whether the stumbling symptoms improve (but be prepared for increased engine ping as a result of it being unplugged).

Safely rig up your pressure gauge to monitor fuel pressure while you drive to determine whether the fuel pressure dips when the symptoms occur.  It should remain constant at 11-12 PSI while the engine is running.

Revisit the secondary ignition.  Thoroughly inspect the spark plug wires for any evidence of brittle and/or grayish spots along the lengths of the wires.  Inspect deep inside the boots at both ends of each wire for greenish discoloration or black carbon deposits.  Ohm out the wires to determine whether any are open or exhibit unusually high resistance.  How many miles have accumulated on the spark plugs?  Remove them and inspect for cracked insulators or carbon tracks, or install a new set.  Inspect inside the distributor cap for cracks and/or carbon tracks.

Dr_Snooz:
Fixing the vacuum leak didn't change anything. Replacing one of the ground straps didn't either. I'm waiting on crimp connectors to replace the other, but don't really expect much there. I swapped the MAP sensor out with the one I had on my 90 C3500. No change. I unplugged the IAC and the idle stopped surging, though I can't say it was great either. I need to test that part more carefully. Forgot to mention that I went through the tests for the EGR. It was getting the right vacuum, opening at the right time, operated smoothly, and killed the engine when actuated. I'll try a test drive without it though to see what happens.

I'm Easter egging, but not really. Just looking at a lot of 35 year old parts that could leave me stranded if they fail. Whether they fix the stumbling or not, I feel better having replaced them.

Dr_Snooz:
I'm back. Sorry, I'm focused on getting my bedroom remodel done and haven't had much time to work on this. I'm waiting for some tools right now, so I had a free day to get back to it.

I hooked up the fuel pressure gauge again. According to the manual, the pressure should reach 9-13psi within 2 seconds of turning the key on. Mine takes 3 seconds, so it's out of spec, but only barely. The strange part is that it sits solidly at 13psi. I idled the truck for awhile and noticed the pressure creep up to 14psi, which seems to coincide with the idle bogging down. I went for a drive and the pressure was steady at 13-14psi throughout the entire drive. It seemed like the stumbling was associated with the drifts up to 14psi, but it's hard to read the gauge without my reading glasses, which I didn't want to put on while driving. At any rate, it's slightly out of spec (or my HF gauge is inaccurate). I was going to proceed with the troubleshooting section in the manual, which should tell me whether I need to replace the pump or the regulator, but my gauge stopped working. So I spent the next couple hours disassembling it and blowing the lines out, then driving for some gas resistant thread sealant to reassemble it again. Gotta wait for the sealant to dry now to see if that fixed it. Everything's a fight.

The next part of the troubleshooting chart is to power the pump directly and see what the pressure does. The manual shows a pin on the ALDL where you can apply power to run the pump. The only problem is that there is no wire in the connector in my truck at the place the manual is showing. SMH. If someone knows another way I can power this pump, I'm all ears.

I had to open the pressure regulator when I replaced the injectors. I kinda had the feeling that I should replace it at that point, sort of like you'd replace any rubber seal you open up, especially after 30 years of service. I wouldn't be surprised if it is failing, but don't know if it's because I opened it, or because it was failing beforehand. In other words, not sure if that's the cause of my problem.

I'm also still waiting on some crimp terminals for the one ground strap I'm replacing. This one runs from the back of the block to the firewall. I'm using 8AWG wire to replace these. Not sure if that's the right size to use. It's what I have on hand. Some of it is leftover house wire, which is really stiff, and some of it is speaker wire, which is really floppy. If there's a song about hillbilly engineering, it's my anthem. The ground strap connection at the firewall is just a course thread bolt drilled directly into the sheet metal. It's clean and sound, but still seems kinda squirrely to me. I'm tempted to replace it with a nutsert. Anyone have an opinion on that?

Thanks everyone for the help.

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