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Discussion Starter #1 (Edited)
Hello gentleman and ladies, hold out for my novel to explain all this.

To introduce myself, I'm a semi-amateur mechanic. I've done all my own repair work since high school and am comfortable doing clutches, headgaskets, etc. I don't know everything and am still learning quite a bit as I fix more things. I have plenty of tools to do almost any job on passenger cars.

I have a 1996 Geo tracker 5 speed, 4wd, 1.6L 16v with 181,xxx miles. Owned for 9 years, bought with 135,000 miles. It's been super reliable until now.

For the past few months, I've noticed an ever so slight stumble with acceleration from a dead stop. I haven't thought much of it until now. I've also noticed that my fuel mileage is a little lower than normal lately (~22 - 24 mpg), but I've just attributed this to the cold winter months. Last summer my mpg was around 28 mpg. The steady misfire problem came on all of a sudden; one morning after startup it had issues and I've been trying to solve it ever since. Currently my car is barely drivable.

These are the codes it's throwing
P0304 (cylinder 4 misfire detected) -- At times this was flashing the check engine light at me while I was getting it home (I know it's bad).
P0420 (catalytic converter below threshold) -- Had this code since the day I bought it.
It had the EGR code awhile back, but it's cleaned now and hasn't come back.

**Symptons**
-idles and runs just fine on cold start and for the first ~4 minutes of run time
-after ~4 minutes of run time, cylinder #4 has a steady misfire when running at idle and below ~1500 rpm
-will run much smoother in neutral above ~1500 rpm
-heavy misfire on acceleration at all rpm ranges while driving.

**Diagnosis done**
-replace spark plugs
-replace distributor cap (got the wrong rotor from the store so it was not replaced)
-test fuel pressure (was good @~38psi, and held steady for 5 minutes for leakdown-no leaks)
-verified misfire was only #4 when I pulled spark plug wire to #4
-compression check was 120 psi on #1 and #4 with WOT. I'm at 4600 feet above sea level, so this would be around 140 psi at sea level. I know this is a bit low, but I have quite a few miles. I was too lazy to do compression test on #2 and #3.
-swap #1 and #4 fuel injector (I thought I had a bad injector, so I swapped #1 and #4) -- cylinder #4 still had misfire.
-swap #3 and #4 spark plug wires to verify spark plug wires were still good (I did swap the wires on the distributor as well) -- still had #4 misfire
-swap fuel injector wire plugs from harness to injectors #3 and #4 (done to verify that computer was sending good signals to #4 injector) -- Still #4 misfire
-Pull out computer and check for bad capacitors -- No leaking/bulging capacitors.
-edit: Also siphoned out all the gas, and put in 1/2 tank of new gas -- still #4 misfire

Recent Repairs:
-timing belt and idler pulley 6 months ago, timing is still good
-oiled up speedometer cable (it was making a chirping noise on cold mornings)
**some oil did leak down onto the wires under the dash**
-spark plugs and distributor cap
-fuel filter 20,000 miles ago
-changed antifreeze 6 months ago

Current thinking:
-not fuel or internal engine related
-I'm thinking it's spark timing related
-ECM has possibly gone bad. It would be awfully hairy to find a replacment, so I haven't looked into it yet
-Distributor may be having issues

I would be very grateful for any experiences similar to what I am experiencing. My next plan of action is to take out the distributor to see if anything is broken inside.

I really like my tracker. It has a proper 4X4 with low range. I love having manual locking hubs so I can use 2wd low range. Very good gas mileage, and the ability to tow light trailers. It's just perfect for me, and I really want to keep it running as long as I can.

Thank you very much for reading this novel as I tried to explain everything!
Russell

I'll respond to any questions or help, but realize the car is not currently at my apartment. They do not allow car repair at my apartment so it is at a friends house right now. I only work on it every 2 or 3 days when I can find time.

Edit: I have searched the forums, and haven't found very much. I did find a few post where 1996+ ECM's have gone bad.
 

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Nice write-up and welcome aboard!

I'm NOT the 1.6 Guru in this model section, but do you not have mechanical lifters requiring periodic valve lash adjustment? :huh:

Too tight a lash on a cold engine won't necessarily kill compression readings, but once warmed and underway, can cause a loss of compression / misfire.
 

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Discussion Starter #3
Nice write-up and welcome aboard!

I'm not the 1.6 Guru in this model section, but do you have mechanical lifters requiring periodic valve lash adjustment? :huh:

Too tight a lash on a cold engine won't necessarily kill compression readings, but once warmed and underway, can cause that misfire.
That's a good point! I did replace 3 of the spring clip keepers for the intake valves lifters 6 months ago when I did the timing belt. I replaced them because one side had broken off. I did adjust all the valve lash to be in tolerance then. This could be the problem, I'll add it to my list to check out.

Edit: Yes, I have mechanical lifters (just to be clear).
 

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Did you re-gap the spark plugs from when they came out of the box. (Pre-Gapped plugs come out of the box with the gap set much too wide.) I THINK they should be gapped to .28 or something. (Bex knows off the top of her head.)

Other than that, my thoughts are to the quality of the new cap and rotor. Do you still have the old ones? (Unless "known bad" I keep my old parts from my last tune up... Just for this type issues.)
 

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Yes, .028" is the correct gap.
And it is unlikely that your ECU is the problem - thus far, at least, the OBD2 computers have not had difficulties.
For the 16v, even at your altitude, your compression is low - it should be about 190psi (sea level) for the 16v. (My car is older than yours, and only the 8v, and I get 175psi across all cylinders). I would do the compression on all cylinders, as well as checking your valve lash, to start. Info on how to do the valve lash, the gap (.006" on all) and the order to do them on the forum.
Additionally, for your replacement rotor, Chinese knock-offs can fit 3 ways - and shouldn't be used. The proper rotor will fit only one way.
I'm also assuming that you are NOT using a Chilton's or Haynes to time this car, as they are full of misinformation. The proper FSM for your car is free to download over on Acks FAQS. Or search the forum here....;)
 

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Discussion Starter #6 (Edited)
Hello Again,

I have checked the valve lash and all were good. Previous spark plugs were gapped correctly, and the new ones are now gapped correctly but it doesn't make a difference.

Here are my compression readings on a cold and hot engine:
#1 #2 #3 #4
Cold Wet 120 140 140 145
Hot Dry 130 130 140 135 (forgot to open throttle, may be ~5 psi low)
Hot Wet 150 155 175 155

Keep in mind that I am at 4500 feet, so divide by 0.87 to get sea level equivalent compression readings. It looks like my rings are somewhat worn, and some my valves have leaks (except #3).

I don't feel that this explains the sudden steady misfire that my vehicle is having.

I still have the old cap. Rotor has not been replaced yet, the correct new one is on the way.

I do have the chilton's but avoid using it. The factory service manual is what I use (when I need to refer to it).

Here is one thing that I have noticed: When hot and engine is running above 1500 rpm, the short term fuel trim is ~0.0%. When hot and idling, the short term fuel increases to about 7% (I can't remember the exact figure). The engine does not miss as often when the short term fuel increases that much. My question is are there any other sensors that could be going bad?

Up next is making sure the timing is correct, and also check out the crank position sensor. Does anyone have any other ideas?

Thanks!

Edit: I remember doing a compression test years ago (probably 9 years ago and 50,000 miles ago), and I vaguely remember that compression was about 150 across all cylinders.
 

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The STFT readings would indicate a vacuum/air leak, any leak tends to have a greater effect at lower loads. Were the spark plugs all the same colour?
I would also check the exhaust manifold for cracks, its a common problem on the 16V
 

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Using your sea level calculation, you should be pulling about 165 psi on all cylinders. I would question whether all rings would show the same wear, and would wonder if your engine timing hasn't jumped a tooth or similar. Have you tried a noid test on the injectors, to see if injector #4 is getting the proper signal? As well as checking its resistance (12 ohms, I think). Do not hook up your injectors directly to 12v, by the way - it destroys them.
 

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Do not hook up your injectors directly to 12v, by the way - it destroys them.
You can run the 16V injectors at 12V for a few seconds without issue. The injector tests given in the Suzuki FSMs state that the injector should be opened for 15 secs at 3 bar pressure. The volume of fuel flowed is then measured and checked against the specification.
 

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Discussion Starter #10
To test the injectors, I swapped injector #1 and #4 and #4 still had a misfire. I also swapped the injector plugs (plugged injector harness connector #3 into injector #4 and vice versa) - Still #4 misfire. That is why I'm leaning along the idea of the spark timing being off for #4. Don't know how to diagnose that though, or if it's even a possibility.

I did look at the manifold, and there is a noticeable crack on the typical #4 exhaust pipe going down into the joint. It wasn't there a year ago. What are the symptons of a leaking exhaust manifold?

I was thinking about buying some exhaust manifold paint to help seal up the crack temporarily to see if that solves the issue. Any advice?

I also just tore it down to the timing belt and it's perfectly on. Has not jumped a tooth. I also checked to make sure the key didn't get messed up. (Edit, the key on the crankshaft to main timing pulley)

I'll also fire it up again and give you some solid numbers on the short term fuel trim at idle and >1500 rpm.
 

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Discussion Starter #11
At my altitude, the minimum compression would be around 148 psi. So I'm not too far off. I don't see it being worth rebuilding at this point. Maybe at a future point when the engine is a little more worn out.

Edit: I'll also check for vacuum leaks and get a vacuum reading to you guys.
 

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Ignition timing will always be the same for each cylinder, its more likely to be a plug lead or bad distributor cap.
 

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I have similar issues with Suzuki Forenza '08 model. I was told that it was either a bad cam sensor or something with the intake manifold.

Notice how the vehicle chugs (although NOT always) when you put your foot on the pedal to accelerate?? This is because the cam sensor is a magnetic piece hooked to a CPU that adjusts the fuel / air mixture to optimum when a change is detected. This is why you can coast and not feel anything.

It will trigger the first piston in line (usually) on older vehicles. This is why older vehicles that are worn get the warning when everything else is OK (my guess only, problem still exists but that is my assumption since every spark plug has been swapped).
 

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I have similar issues with Suzuki Forenza '08 model. I was told that it was either a bad cam sensor or something with the intake manifold.
There is a Firenze forum in the links on the left of this page. An entirely different car and generation from this side of the forum....:)
 
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