Hard starting, low power, surging idle... I'm at a loss

iamnatman

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This is for a 92 RMW with the L05 TBI 350.

I've been dealing with this problem for about 4 months now, and I have no clue what keeps causing this issue. I've swapped the fuel pump, fuel filter (twice), fuel pressure regulator, throttle body gaskets, and *idle air control valve*. I had my mechanic look at it, and he told me the fuel system was fine and has the right amount of pressure behind it. He was thinking the catalytic converter was to blame. Sure enough, it was pretty clogged up, but having it off of the vehicle and straight piping still is causing this annoying problem.

If I let it idle for about 5 minutes with a bit of revving here and there, it will eventually stabilize and stop surging in RPMs. However, there is a serious lack of power and I'm barely able to move it around on the road. I'm not noticing any kind of misfire, and the plugs/wires are not that old.

My mechanic had told me upon putting a new converter on, the IAC would "relearn" everything and should stop the idle surging. Before I go dropping more money into this beast, I wondered if anyone has had similar symptoms and if there's something I might be overlooking?

Thank you.
 
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Dirtybob

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I would verify that the egr valve is not sticking open and check the timing. Should be 0 degrees with the est connector unplugged (supposedly a tan/white 2 wire connector on pass side of the intake manifold).
I've seen a few poor running tbi motors where someone tried to fiddle with the timing but did not unplug the est first....
 

iamnatman

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I would verify that the egr valve is not sticking open and check the timing. Should be 0 degrees with the est connector unplugged (supposedly a tan/white 2 wire connector on pass side of the intake manifold).
I've seen a few poor running tbi motors where someone tried to fiddle with the timing but did not unplug the est first....
Thanks Bob!

Yeah, I wondered if the timing was to blame. I didn't have much time earlier, but I pulled the dist. cap off earlier and noticed an alarming amount of rust and corrosion on the rotor assembly. I saw too that the ICM had a "GM" logo on top, which tells me that all of those components are likely factory, and the engine is at 210k miles. Now, I know the block itself was swapped with a reman one at 190k (before I bought it), but I'm betting the shop just reused most of the original components.

For that reason alone, I might swap the whole distributor/ICM out and see what happens. Whether that's the source or not, it doesn't look great.
 

iamnatman

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Sorry, I just realized I said I swapped out the ICM in the first post. It was actually the idle air control valve I changed, NOT the ICM.
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Dirtybob

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I have seen the distributor gear wear out on a high mile vortec 350 (250k miles on it). The teeth were worn paper thin and the rotor had excessive play, easy to check w/o even pulling the distributor. Much over 1/16" free play when turning the rotor and I would pull it and inspect. If the distributor looks ok you should still thow a new cap and rotor on it and get the ICM tested. Make sure they run the test multiple times - until the ICM gets good and hot.
 

iamnatman

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Swapped the cap, rotor and ICM out. Sadly that didn't fix the issue, even though all components were in pretty rough shape. I didn't notice much play on the distributor shaft when I had the cap off. I'm gonna check the EGR next, but it looks like those bolts are a little tricky to get to with a regular socket. They also seem like a weird size in between 12 and 13mm. Even my 1/2 wrench seemed a little too large with a considerable amount of play for the bolt. Hopefully I can get them off without stripping anything.
 

iamnatman

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Alright yeah, I just checked the EGR still on the car, and it's not holding a vacuum. Hoping this is the issue now. I'm going to soak those bolts overnight and pray they don't strip.
 
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#8
I wonder if you can get a torch on the EGR nuts. Torch would help.
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EGR actually came off pretty easily. Sadly, after putting a new one in (even with the old one being completely broken), it's still surging in idle like before.

However, I pulled the vacuum line going to the MAP sensor... and it cleared right up. With the line disconnected, the idle is where it needs to be and throttle response seems normal. Plugging the open vacuum line will drop the idle speed slightly, but the moment I hook it back up to the sensor, it surges up and down.

Is it safe to say at this point the MAP is probably shot?
 
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