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Highway Speed "Bucking"/Misfire?; diagnostic troubleshooting
Topic Started: Oct 12 2017, 12:29 PM (335 Views)
MattyG
Fresh Fish
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Hey y'all, I'm new to the tribe having bought my first 1997 Geo Metro a couple weeks ago. I'm entirely in love with the little thing already and have been using this as an opportunity to verse myself in the grease-world of engine mechanics. I've done a fair bit of searching on this site and otherwise as to what problem my Geo could be experiencing.

So far, the problem that keeps recurring is when I take the Geo on the highway and get it up to 55-60mph+ AND/OR am putting strain on it going up an incline, it begins to "buck" while constant pressure is remaining on the acceleration pedal. The way I can describe this bucking is it feels like either gas isn't getting to the engine, or the cylinders are misfiring... The weirdest parts about it are 1) it doesn't happen every time I get on the highway and 2) if I downshift it sometimes solves the problem temporarily and lets me drive in low gear/low speed (in 4th gear at about 40-45 mph). The car drives great around town with no problems if I stay away from the freeway. Additionally, this bucking problem has only occurred after driving on the highway for some time; it has happened after 20 min, and even once after an hour and a half. Usually, I just pull off the highway, let the car chill for a little, and then keep driving at a somewhat reduced speed.

I've been doing what I can to try and diagnose the problem so far. I did a compression test not too long ago and came up with these numbers: cyl 1: 130 cyl 2: 175 cyl 3: 180
Now I used this reference to complete the test: https://www.youtube.com/watch?v=_MPTJJznNbc
I got some pretty weird values at first and did the test a couple more times and came up with these stable numbers.

Additionally, I was planning on cleaning the EGR and replacing the gasket sometime soon. I don't really have any reason for doing this besides the fact that I can do it myself easily, and perhaps it will somehow fix the problem (as you can see I'm really just shooting in the dark about this)

Next, I noticed a somewhat big hole in my RADIATOR... That can't be good. So I was gonna order one of those and replace it because it definitely needs it. Could this be the problem for my "bucking"?

Also, somewhat unrelated but good information: new battery; new spark plugs; I checked the air filter and it seems theres some oil in there; new oil/filter; new fuel filter.


Finally, my speculation is this: Could it possibly be a coolant system problem? I do leak coolant very slowly (a couple splashes every now and then) and my temperature gauge runs a little bit on the hot side of halfway. Or could it be a fuel delivery problem? I had the fuel filter replaced after the first bucking incident--it didn't happen again for a while (no idea if cause or just correlation). Either problem makes some sense to me because at high speed I imagine any flaw to either system would be exacerbated.

So let me know what you all think! I really appreciate any time you take to answer my dilemma. Thank you!

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ptcapboy


the first thing that comes to my mind is the distributer-
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Azhule
Elite Member
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Check, clean and or replace the PCV if you find oil in the air filter, might have a stuck open valve... if you already replaced the (PCV) valve recently, you might need to replace it again with a different brand name/OE brand part of higher quality, sometimes the springs and other parts in the cheaper ones don't seal properly or have the correct pressure to "open and close" properly causing problems
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freegeo
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When doing a compression test it is important to remember to disable the fuel injector. I pull the FI fuse. Spark needs disabled. I disconnect the connector at the coil.

When cranking the engine over the throttle needs to be held wide open. If it is not it can lower the readings.

I do not agree with what Geo Jeff says about putting oil in the cylinders. If the results are lower then the factory minimum then adding oil to the cylinder can help evaluate if the cause is due to the rings or valves.

Since the problem is not constant, start with the basics.

Fuel pressure at the throttle body.
Ignition timing - set it according to the emission sticker.

It could also be a electrical issue too.
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JellyBeanDriver


Check your spark plug gap. If it's wide it will do that. Also if your ignition system is weak.
Edited by JellyBeanDriver, Oct 12 2017, 11:12 PM.
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Dystopiate666
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I saw a night and day change the first time I did a full tune up on my geo....the previous owner was a firm believer that 5000 miles was fine for oil changes if the driving was all pavement, he talked about washing his air filter with water, plug tips burnt to a point....etc....

Do the plugs, wires, cap, rotor....air and fuel filters you said you got already...then verify everything is good...fuel pressure, timing, and fix the radiator and wherever else the car might be leaking so you can determine how much oil and coolant the little car is actually consuming and how much of it is falling on the ground....Assuming that you NEVER let it get low enough to overheat, the hole in your radiator alone wont cause your issues, if it starts displaying these symptoms from an overheat, it might be time to start sourcing parts for your rebuild... I suspect your problem is elsewhere, however, playing with your coolant system is asking for trouble...fix those leaks and make sure your gauge is working properly.

A little oil on the underside of the air filter isn't much to worry about, it is a sign that your engine is getting worn and happens for a variety or combination of reasons including blowby from tired rings, worn cam journals, or failed PCV, mine has gone a long time without any noticeable change in amount of oil it deposits over the last 5 years or so....however, if it starts to pool on the plastic, or burn holes in the filter, or blow huge smokes clouds out the tailpipe, that would be considered excessive by some (however not unfixable and the car will still start and drive with those conditions for some time)...

Its all about getting to ground level with a new car, one thing at a time and pretty soon you are driving a reliable little gas sipper.
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Moringa
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Living BOT

Hi, and sorry about your situation. The following part of your initial thread sounded very familiar:
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Additionally, this bucking problem has only occurred after driving on the highway for some time; it has happened after 20 min, and even once after an hour and a half. Usually, I just pull off the highway, let the car chill for a little, and then keep driving at a somewhat reduced speed.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
I used to have a vehicle that did exactly this. It turned out to be the fuel tank strainer was accumulating crud, and getting so plugged that the fuel pump couldn't suck gas through it. Most, if not all, in tank fuel pumps have a strainer on them, to keep them from sucking up large particles, and ruining the pump. At it's worst, my vehicle would be cruising on the freeway for 4 or 5 miles, then start to buck and die. I'd pull over and have to wait for a half hour, then it would restart, and I could go another 4 or 5 miles. I would think that fuel tank crud is the culprit.
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MattyG
Fresh Fish
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Thanks everyone for the responses! I think I've got a good enough to-do list with all of your suggestions, I'll update once I've gone through some of them.
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Stubby79
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JellyBeanDriver
Oct 12 2017, 11:11 PM
Check your spark plug gap. If it's wide it will do that.
This is what it was for me. (Different car, mind you, but same issue...though I had an easier time of reproducing it). Spark plugs had worn out prematurely.
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3 Pot
3 Pot
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I had a very similar problem with my 1993 3 Cyl 5 Speed. It is a older, no where new engine but still a very drivable car. The problem was that the o ring in the distributor was starting to fail and thru a very fine film of oil around the distributor cap and rotor to the point where it started to buck on higher loads but would run fine under lower loads. I good cleaning with brake or electrical cleaner of the cap and rotor (or replacement) might fix you up as it did for me. No issues after that for... well, it's still working fine.

3 Pot
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PTA2PTB
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I'm totally awesome! I swear.

MattyG
Oct 12 2017, 12:29 PM
So far, the problem that keeps recurring is when I take the Geo on the highway and get it up to 55-60mph+ AND/OR am putting strain on it going up an incline, it begins to "buck" while constant pressure is remaining on the acceleration pedal. The way I can describe this bucking is it feels like either gas isn't getting to the engine, or the cylinders are misfiring... The weirdest parts about it are 1) it doesn't happen every time I get on the highway and 2) if I downshift it sometimes solves the problem temporarily and lets me drive in low gear/low speed (in 4th gear at about 40-45 mph). The car drives great around town with no problems if I stay away from the freeway. Additionally, this bucking problem has only occurred after driving on the highway for some time; it has happened after 20 min, and even once after an hour and a half. Usually, I just pull off the highway, let the car chill for a little, and then keep driving at a somewhat reduced speed.

Based on my similar experiences with my car, that sounds like classic TPS being bad or in need of adjustment.
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Car Nut
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Take a few minutes and read through this thread that i posted a couple years ago. Exact same issue you described, even the downshifting part. Turned out to be the fuel injector. Let us know what you figure out.

http://geometroforum.com/topic/5855090/1/
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