sammarx22

1996 2.0L Starts And Idles Perfectly, But As Soon As You Touch The Gas

22 posts in this topic

Hello all, 

 

I've searched as extensively as I could and was unable to find the same issue as I am having.  My 96 Mazda 626 2.0L Manual will start perfectly and idle perfectly, but as soon as I touch the gas pedal (or manually try to open the throttle at the throttle body) the car completely dies.  I've tried feathering it open but as soon as the throttle leaves the idle position, the car dies.  If I quickly hold the throttle wide open from idle it will just barely run (fires maybe once per second) and back fires dramatically with smoke and the works.  I back probed the pins on the TPS and saw 4.5 volts from the reference, and at idle it showed 0.6 volts at the signal pin.  As I opened the throttle the signal voltage dropped to 0 volts, so thought it was the throttle position sensor and replaced it.  After replacing the TPS it does the same exact thing.  I'm fairly certain the TPS is working because if I move it like I'm adjusting it the idle will change, but the issue of the engine stalling as soon as the throttle is opened remains.  I tried checking the signal voltage again, but my multi-meter seems to have died and my good one is at work, so I will check that again tomorrow.  While I had the intake tubing off, I replaced the fuel filter.  I know the fuel pump is working because I left the outlet hose off the fuel filter and bumped the engine over and fuel shot into the air.  Within the last year (5,000 miles) I replaced the timing belt/water pump, removed the throttle body and cleaned it and put a good used IAC valve on it, new exhaust (minus the cat, which the muffler guy said was clear), and feel like I did more, but can't remember right now.  Are there any other sensors for the throttle that could cause this issue?  Or maybe the PCM/ECU took a dive?  Any advice would be great, and if I've missed a post that has my answer, I sincerely apologize. 

 

Thanks in advance!

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Sounds like the timing belt might have jumped a tooth.  Inspect your mechanical timing.  Vacuum leak combined with bad timing will make your car sound and run like hell in a hand basket.  If it was just a vacuum leak you should be able to very very slow raise the engine RPM up without it stalling.  The higher the RPM the less effect a vacuum leak should have (it should run better at higher rpm if you can get it there).

 

Also just because fuel shoots into the air does not mean you have good fuel pressure.  You must get a fuel pressure tester to correctly test the fuel pump.  Fuel volume means nothing.  Fuel pressure means everything.

 

 

I'm a huge supporter of that video because it's exactly what happened to my 626.  I found the issue with a fuel pressure test and eventually traced the issue back to the fuel tank.  My diagnostic steps worked perfectly and voila there was the issue.

 

https://youtu.be/eeSdKNEMdwg?t=3m

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For me it sounds like the MAF is unplugged or not working, I've had the same problems several times because I simply forgot to plug it in after doing something, car would run ok, but would die if I gave it gas. And the worst thing is you feel stupid every time for forgetting to plug it back in.

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So I was finally able to get back to the little Mazda.  I replaced the timing belt/water pump last fall, but went ahead and took things off to check the timing, and it still looked good. I had some funny readings on my crank shaft sensor, so I replaced that, along with spark plugs and wires.  I did a fuel pressure test and it was around 25 or 30 psi just by turning ignition on and off, and close to 40 psi when the car is running.  The fuel pressure held just fine for quite some time, but I can check that again to see just how long it holds at a constant pressure to verify that the fuel pressure regulator is ok.  I rattled the catalytic converter and didn't hear anything rattling inside, and exhaust still comes out the tail pipe so I don't think that's the problem.  

As I stated before, my car would start and idle perfectly, but as soon as the throttle was touched it would spit, sputter, misfire, and eventually die.  Now, after a mistake of leaving a rag in the throttle body intake and starting the car (throttle plate was closed so the rag wasn't consumed), the car starts, idles, and if the throttle is applied very slowly it will rev all the way to red line.  If I give it throttle quickly, it still misfires and spits like mad and won't rev above whatever rpm it's at when WOT is applied, unless I back the throttle off and then slowly re-apply.  This makes me thing it's something air related (blockage of air opened something up allowing air/fuel to pass through somewhat correctly).  I sprayed electric contact  cleaner (the flammable stuff) around the throttle body gasket, and all around the intake manifold hoping that the car idle would change (meaning a leak) but nothing.  I can go ahead and pull the throttle body and intake manifold off and replace all the gaskets, but I'm just not sure I'm chasing the right issue.  

My other thought would be maybe a coil issue?  But I think the coil on my car (96 2.0L w/manual trans) is in the distributor, so that would mean replacing the entire dist?

I can try to post a video later of what it's doing if that might help or my explanation doesn't make any sense. 

Any advice is welcome.  Thanks!

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if the throttle is applied very slowly it will rev all the way to red line.  If I give it throttle quickly, it still misfires and spits like mad

This bit of information is extremely helpful.  This tells us it's not mechanical or ignition timing related. It's a classic sign of a vacuum leak.  That test has just helped rule out timing, TPS, and O2 sensor in 1 shot.  Good job.  It's most likely a vacuum leak now.  A bad exhaust manifold leak prior to the O2 sensor will do the same thing but you would see the smoke rising from the exhaust manifold, that one should be kind of obvious.  Carb cleaner isn't a definitive test for a vacuum leak, a smoke machine is.  Check all of your vacuum hoses and evap routing too.  I had a vacuum leak at a vacuum Tee on the backside of the intake manifold very hard to see.  The rubber cap was missing from the port and it caused all kinds of havoc with my idle and throttle response.  Smoke test is the best method, you can take it to a shop that has a smoke machine and get them to find the leak for you.

 

It's not an air blockage.  If it was then you wouldn't be able to go all the way up to higher RPM's slowly.  You would hit a certain RPM and be unable to go above it due to air restriction.  You've proved there isn't an air restriction, in fact just the opposite, there's too much unmetered air.

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I did a vacuum test with a homemade smoke machine. It was a small 12v air compressor into a basketball needle into a jar with some of those fire works lighters, then out of a 3/8 hose into a sealed cap on the intake.  I took the MAF off and put the sealed cap right into the rubber of the intake (to make sure the rubber intake tube didn't have any cracks).  This setup gave me a nice low velocity smoke with good pressure to get into all the nooks and crannies of the intake.  

Well anyways, I let that run for quite a while and slowly went all around the intake to look for smoke rising.  I was in the garage with all fans off and was breathing very lightly.  I looked for probably 15 minutes, and the only smoke I saw was a very slight amount coming from the dipstick (the o-ring didn't quite seal all the way).  And a very tiny amount from what looked like somewhere around the throttle body.  I pulled the throttle body off and the gasket doesn't look like it was bad.  

So I have two questions...  

1. Would that be enough?

2.  What is the small vacuum chamber looking deal right below the throttle body?  

 

The way this whole deal originally happened, I was convinced it was electrical.  There's a small electrical connector going to that thing, so wondering if something there is wrong, and also want to look into the evap purge solenoid.  I know a few youtube videos say that shouldn't cause as bad of an issue as I was having, but still wonder if it could be part of the cause.

 

Thanks!

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Smoke coming out of the dipstick could be a sign that you have blown piston rings.  There will be a little that escapes passed the rings.  Only way to know for sure if it's a big problem is a compression test which at this point should be your next step  Yes compression loss could be mistaken for a vacuum leak in terms of power loss

 

The small vacuum chamber below the throttle body is the air resonator.  It's like a muffler for the intake system.

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The dipstick connects to the IM via the PCV, I doubt it has anything to do with the rings.

 

The other possibility is your fuel pressure regulator.  Your fuel pump may be working fine, but if the regulator is stuck open, it will idle fine, but won't deliver enough fuel to accelerate the engine.  It chokes because you open the throttle and a bunch of extra air comes in, but the fuel pressure doesn't increase like it's supposed to, so not enough fuel.

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Thanks everyone for the awesome help so far.  I feel like I'm chasing a ghost through this car.  And because I primarily run diesel pickups, I'm not used to all the emissions stuff on this car.  

 

This morning I cleaned the throttle body, cleaned the EGR valve, checked the purge solenoid and pulled off the fuel pressure regulator.  EGR valve looked fairly good for having 180,000 miles on it.  Purge solenoid worked properly when 12v was applied, and the tubes seemed clear.  But when I pulled off the pressure regulator line all the way on the passenger side, there was gas in the vacuum line.  When i pulled the actual regulator off, I applied a vacuum to where the vacuum line connects and the vacuum chamber was filled with gas.  Now I'm assuming that's not supposed to be like that, so am guessing that the whatever the media is within the pressure regulator is bad, therefore allowing gas to go into the vacuum line.  Could this possibly be the cause of all of my grief?

 

Thanks!

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Well I was finally able to find time to take a look at the 626 again. I replaced the fuel pressure regulator and it had no effect. So I pulled off the vacuum solenoids for the egr and pressure regulator and tested them and checked all the vacuum lines and everything looked good.

I've noticed that if I start the car with the intake tube off and hold my hand over it so only a small portion is exposed I can rev the engine up with little to no hesitation. So this tells me once again that either there is too much air getting in, or not enough fuel getting in (performed both smoke test for vacuum leaks and fuel pressure test and everything checked out good).

Another thing I noticed is if I pull the connector off for the coolant temp sensor, the car runs better (fan kicks on, idle raises, and I can rev it a little easier and it feels like it has more power under acceleration). So could a coolant temp sensor be an issue, or is the ECU just defaulting to some kind of base map (like more fuel) and that's why it runs a little better?

Even though my fuel pump runs and builds pressure well, could it be an issue of pressure vs flow? Like a partially plugged line, or intermitant operation?

Im just trying to figure out what's going on and really don't feel like spending money on the spare car.

Thanks again.

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Sorry to redirect this post again. I was tinkering with the Mazda again thus weekend. I rechecked fuel pressure and did a flow test (out of the filter) and had a maz pressure of 67 psi, and flow rate of 44.5 gph. I also did the testbin thebhaynes manual and it checked out fine. So I've ruled out fuel pump. I decided it was easy enough to pull the intake manifold off, so did that and checked everything there (no cracks, all sorts were clear, removed hex but and cleaned that but it wasn't plugged and all vacuum lines were clear and looked good). The other thing I noticed was the oil pressure sending unit wire was broke off and dangling. But my oil light hasn't lit up. Is the oil pressure tied into the ECU? That it might cause a limp mode of sorts? This is a manual transmission, so am not sure if it even has a limp mode.

The other thing to note was that when I was flow testing the gas, I was using a glass jar and noticed that it is pretty bad gasoline (yellow and smells like varnish) so maybe just a bad batch if gas? I've got everything back together now and just need to drain the gas, add fresh and see what happens. Thanks again for any input.

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I still think the MAF is bad man. Or it could be bad plug wires, had that problem myself where the idle was ok except for under load.

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MAF tested out, and I hate replacing parts that say they're working. And it has new plug wires.

Ditto, but if you got a spare a crosswap is an easy way to figure out if it's bad or not.

But I still think it's something MAF related, the 96 should have it's own IAT sensor, have you checked it? And see if you get 12v to the MAF, kinda sounds like you don't.

New plug wires is good, what about the plugs?

The only other thing that could come to my mind is a clogged cat.

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Just went through this last fall, if your 96 2.0 has a distributor still, thats the culprit, its actually just the little capacitor under the distributor body.

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Parts telling you they are ok, can still be defective.
A sensor that's of his factory settings, will not necessarily pull a fault code on your ECU, but it can cause a lot of problems.

Especcialy mafs are known to be bad but not pulling a fault on your ECU.

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Did you ever do a compression test? It's free to do (with a tool rental from a local parts store) & will give you reassurance that your ghost isn't a mechanical problem with the valves or rings.

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My MAF is bad. Have the same symptoms.  It shows power but when opening the voltage should drop.  Not a lot could cause it.  IAC, maf and tps. If everything else works. 

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Ended up being the distributor... $62 part on ebay and it was back in action.

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Wow, good thing you figured it out.

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