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Another "loss of power" thread; But this one's mine ;)
Topic Started: Nov 20 2016, 01:04 PM (858 Views)
scratchpaddy
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Not so fast

A few months ago, my Geo ('96 3/5) started acting up just after starting. It has no issue starting (besides the occasional "starter click" issue, but that's unrelated), but then the idle intermittently drops. If I let it sit there, it sometimes stalls. If I get going right away, it bucks in first gear, like a beginner driving stick. This continues for about 30 seconds, sometimes less. Then, it's full speed ahead. Idle and vacuum are steady after that.

It almost always happens when the engine is still halfway warm, like when I come out from the grocery store, guaranteeing difficult parking lot exits. It happens maybe half the time after a cold start.

Yesterday, it continued to act up for an entire 10-mile drive, starting from cold. It stalled twice at red lights, with me just sitting there, out of gear, foot off the pedals. It randomly refused to accelerate, but never lost power enough to lose speed. Flooring the gas pedal made no difference, but backing off to about quarter-throttle gave it more power. I drove it 40 miles this morning, and it was back to messing up for 30 seconds, followed by normal performance for the rest of the drive.

The CEL comes on once in a while with P0400 (EGR). It's been doing this since I bought the car 18 months ago. The light has been off for the past several hundred miles, but the EGR and O² sensor diagnostic checks still show "incomplete" after all this time.

Compression is 203-190-201, which is strange, because the original compression test I did after getting the car said 182-168-181. I installed an economy-grind camshaft since then, does that boost compression? Or is my gage messed up? I installed the camshaft a year before this power loss issue came up.

Any ideas? Thanks in advance. :)

By the way, I'm planning on selling the car soon, but I don't want to dump problems on the new owner. :shake
Edited by scratchpaddy, Nov 20 2016, 01:07 PM.
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evmetro


I would do a vacuum test.
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scratchpaddy
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Not so fast

What's involved in a vacuum test?

edit: at idle, it fluctuates back and forth from 18 to 20 mm inHg. Is that the info you're looking for? Is that bad? I think it's always been like this.
Edited by scratchpaddy, Nov 21 2016, 05:25 PM.
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MR1 Kingsbury
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Vac and compression seem good. sounds like a fuel delivery thing. You can try a 5 dollar bottle of injector cleaner product and see if symptoms change. Not likely a fuel filter but could be pump, or injector.
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scratchpaddy
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Fuel was my first thought as well. I did use one of those miracle bottles on the last tank just for kicks, but even if the injector was the issue, I would be surprised if a bottle in the tank fixed it. I replaced the fuel injector a while ago, but it was a junkyard unit. Fuel filter was replaced a year ago. Sounds like I should test the fuel pressure. Looks like Autozone has a test kit I can borrow.

I replaced every vacuum line more than a year ago, but I might still have some kind of vacuum problem, since the EGR code keeps coming up.

Could the catalytic converter also cause these symptoms? It's original to the car.
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MR1 Kingsbury
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As a matter of fact, I think the cat could cause this issue.. if it is busted up inside the pieces could rattle around and restrict exhaust. smack your cat a bit and see if its loose inside. if not than this isn't likely the culprit. Is the cel on now? after you start it?
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scratchpaddy
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Not so fast

No check engine light right now. I smacked the cat around as much as I could with it still on the car, but didn't hear any rattles.

I started it again today to test, and the idle started falling every second or two, like it does when it decides to act up. Vacuum would drop to 15 inHg whenever it stumbled. After it stabilized, I pressed the gas, and vacuum rose slightly (to 21 inHg) until I passed 2,000 RPM. At that point, it was like hitting a wall. RPM fell even when I gave it more gas, and vacuum dropped to 14 inHg. When I took my foot off the gas, it went back to idling normally.

The exhaust upstream from the cat was too hot to touch after this, but the pipe after the cat was only warm. I guess now I need to disconnect the exhaust and see how it acts. It's gonna get loud... :scared
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MR1 Kingsbury
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do you have exhaust pushing out of the tailpipe? or just a little pressure there. you can remove the front O2 sensor for a little less exhaust pressure and see how that acts. It does seem like an exhaust restriction but it could be a mal adjusted throttle position sensor or a failing map sensor.
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freegeo
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You don't want to have the O2 sensor out of the exhaust and plugged in. It will drive the fuel trims crazy.
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scratchpaddy
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There is exhaust coming out the tailpipe, but it doesn't feel like it has much pressure to it. I'm not sure how much pressure it's supposed to have, though.

Before I disconnected the exhaust, I did the same test again, and it acted completely normal, with steady, high vacuum even to 3,000+ RPM. So, I decided not to test without the exhaust connected. What would it prove? Sometimes it works normally without me changing anything. :dunno

The vacuum readings I've been quoting have been from the MAP sensor, through an OBDII scanner. I don't think it's failing, since all the readings it's been giving me make sense. When the engine loses power, rather than feeling like it's trying less hard, it gets louder, and feels like it's working harder, but for some reason not getting as much done. That's basically what it means when the vacuum drops, right? That the engine is working harder.

I'll figure out how to test the TPS tomorrow. If that's fine, I'll pull the cat so I can shake it properly. It's hard to tell if anything's loose in there when it's bolted in to the rest of the system.

Thanks for the suggestions so far.

freegeo
Nov 21 2016, 10:08 PM
You don't want to have the O2 sensor out of the exhaust and plugged in. It will drive the fuel trims crazy.

He's not saying I should take it out permanently, but just temporarily to open a hole for exhaust to escape before the cat.
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MR1 Kingsbury
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The whole intermittent thing puzzles me a bit... Not likely an exhaust clog... It is beginning to mock a failing fuel injector... they have all sorts of weird failing symptoms, some will plain quit and strand you for an hour or two, sometimes they give you just enough fuel to idle and little more.
The only other thing I can think of is a bad crank gear key that allows the timing to slip back and forth... I had that for about 3 days once until it broke a piston...Long story there, but the key failure didn't break the piston... the nut that used to hold the air cleaner on fell in and got into a cylinder.
watch the front crank pulley and notice whether it wobbles at all. If it doesn't, I would be chasing the fuel gremlin. Look down the intake while it is faulting and see if there is a steady cone of fuel sprayin in.
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scratchpaddy
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Ok, I'll check for spray next time it acts up. When my original injector started failing, it either worked perfectly, or not at all. Wouldn't it be funny if the injector gets me twice? :lol

There's no wobble at all in the crank pulley. I've never taken the crank timing gear off before.

This car has 259,638 miles, by the way. The previous owner had the motor rebuilt around 240k, and judging by the state of the car when I got it, he never did any repair unless it was absolutely required. That's why I'm suspicious of the cat, which was definitely not replaced.

I'm still going to check the TPS, that seems important. I get the feeler gage part, but then the FSM says to "backprobe the PCM on the TP sensor signal circuit to ground." I'm not sure where it's asking me to probe? I can't find any information on which wires and pins do what, at least in this section. I'm on page 6E2-C1-14, Book 2, 1996 FSM.
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scratchpaddy
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I watched inside the throttle body during a stumbling episode, and it didn't look like the fuel supply ever stopped. I couldn't find the right angle for the light to be able to see the actual cone of fuel, but I could see the gas hitting the butterfly plate.

I found a picture (here) showing exactly where to probe the TPS. Mine was way off, 1.31V with .14" shim between the stop screw and throttle. I adjusted it down to 1.02V, started the car, and the ISC started going nuts, running the engine from 1,500 to 2k RPM. Even after the engine warmed up, it never dropped below 1,500 RPM, and it kept bumping up to 2k every few seconds.

I'm thinking my throttle stop screw was messed with, and maybe the TPS was in the right spot all along. If that's the case, how do I know where to set the stop screw? I guess I could adjust the TPS back to 1.31V, then back off the stop screw until it reads 1.00V with the .14" shim in there. Or has my train of thought run off the rails?
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freegeo
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You need to make sure you follow the fsm procedure for this. I used that how to when I got my car. Had it all screwed up like yours is. The engine has to be warmed up and the ISC can't be touching the throttle.

If you need the procedure I can post it.
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scratchpaddy
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I've read it's "the screw which shall not be touched." :lol I have the '96 FSM, but if you could let me know where to look, that would be a big help. I can never find anything in these books. :dunno
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