Lying Dealership says compression is the issue
 

Are these numbers really that low? Can't find anything here that has numbers like these. Car stutters at high rpm (above 5K) and seems really weak at high RPMs (above 5k), I've posted about it before and was told to get the car compression tested. Just got one today at the dealer and:

7.3
7.4
7.3
7.5
7.6
7.4

they also said this is why it runs poorly and I have to replace the engine to fix the issues. I realize if it's really below spec, then it will need to be replaced eventually, but I do not think it's causing the stuttering. Always starts right up, too. And runs great at low rpms. I told the guy that compression issues start at low RPMs but he said that it's the issue and nothing else. Also had the fuel pressure tested--60psi. Have already replaced ignition, and cleaned all sensors.

Failing cat?

What rpm was the test performed at?

A clogged cat will give those symptoms - it's NOT a compression problem!

An engine with zero compression and blown out apex seals will run quite well at high revs. Been there, T-shirt acquired.

Borrow a 'test pipe' to see if it runs well with no cat (or loosen the cat/header joint to let the gasses out - you'll need ear plugs!)

If it runs well bypassing the cat you can get a new one(!) or an aftermarket, or just keep a test pipe in, depending on the requirements in Tennessee.

The cat is not the issue, as the insides have been cleared out by the previous owner (it was the first thing I checked). and I don't know what RPM it was done at. I didn't ask them to do the test, they just did it and gave me the results. They said that mazda spec was 8.0 though and that the engine was the issue. I argued but he just said that was it. So I left with the car. I have already replaced all the plugs, coils, and wires, cleaned the MAF, replaced the ESS (faulty), done a seafoam, and had an intake valve unstuck. And had the fuel pressure tested==60psi. All I have now is P0300 and I'm stumped

Fuel pump? Injectors?

My next 'guess' would be the sock filter in the fuel pump housing.

At least it's a no-cost check/fix.....

It's not the pump because the pressure tested 60psi, and I don't see how it would be the injectors, unless they go bad. I've already run fuel system cleaner through the system. I should check the sock though, good point. I'll see if I can't find one of those ring tool things

Cleaned the fuel sock today and it got better, but still is weak above 5k. The car literally shudders lightly. The engine pulls hard, weakens, hard, weakens, hard, weakens, (as the RPMs climb) and sometimes above 6k in 4th it pulls like it should....really hard! I don't know...maybe the dealer lied about the fuel pressure?

Mazda compression "spec" of 8.0 is the goal of where an engine should be when new. It fails when it hits 5.9 at 250rpm. You only fail compression if that is at ~350-400rpm, which is higher than our starters spin. As noted, compression is not your problem.


I applaud you for working through the typical list already. If you hadn't, I'd point at an ignition failure.

Almost sounds like an intake valve that isn't reacting normally. It's possible that the actuator is failing, or you have a vacuum leak in the line that runs to one of the valves.

The other thing I'd start looking at is if you have clean solid grounds, primarily the engine block ground. If your ground is corroding up, it could have trouble producing a spark at the spark plugs.

Can you pull your plugs? Should be an indication of if you are running rich, which I suspect is the case. If it's a fueling problem you should be running lean. A spark problem will have you running rich. Plugs can help give the info to know which way you are going. If you have an ODB2 reader that can check the AFRs, that will be good backup info. Actually a primary O2 sensor failing might cause this as well, as it might keep switching up the signals that it's sending to the ECU and the ECU is just following along not realizing that it's doing something wrong.

I had a code saying that my intake runner control valve was stuck open, and I had a local rotary guy do a de-carbon in the intake and the code went away, so I don't suppose that could still be the problem.....? What would I be looking for in the spark plugs that would give me an answer on lean vs rich? The ground connection on my battery is tight, but I will check the other end. I also have a second ground the rotary guy put on that runs from the ground battery terminal to the block. I'll also see about getting the valves and vacuum lines checked because I have no clue where that stuff is located lol. And would the AFRs be the fuel ratios?

Thanks!!

Checking the spark plugs is the classic method to determine lean or rich. When you run rich, they will get black and carbon coated, when you run lean they will get whiter. Now, when your engine is running just fine, our plugs turn black anyway, because in general we run quite rich. But a problem that would make you run lean should start showing up as whiteness on your plugs.

Yes, AFR is Air to Fuel Ratio. It's reported from the front O2 sensor and used to try to keep the engine running at the target ratio. If it detects lean, it adds fuel, if it detects rich it pulls fuel. If it's failing, then it could be detecting more rich or more lean than it actually is, or the opposite of what it is, or doesn't really report lean or doesn't really report rich, and this can start playing havoc with the ECU's attempts to fuel the engine, before the ECU even picks up on whether or not the sensor might be bad.

Intake valve debug: Search for posts by Jon316G about SSV and APV. A vacuum test manual pump is a good tool for the debug process.

Consider a Cobb Access Port as it provides very good data logging and can be used to enable or disable various intake valves for troubleshooting.

I agree it could be the SSV valve sticking... you can try moving it by hand to see if it is sticking/stuck... my 04 WB I used to have the SSV valve was stuck to where you could not even move it by hand... like suggested look for the DIY to remove/clean the SSV if necessary...

I've done some searching and I need to manually operate the ssv to see if it feels sticky, and I've found posts telling to do this, but none tell where the ssv is located/what to open or close to test it while it's still in the engine. So can one of you guys describe where it is? I would rather move it and see than remove it, unless it really is sticking.

I also will see if my buddy's code reader can determine AFRs, as mine is only a basic code reader :/
I also recently replaced the plugs, so would they have turned black or white yet since I've only driven 1000 miles or a bit less since they were replaced?

Thanks

Yes, it can start pretty quick. If you had 10-20k on those plugs, it would be a lot harder to determine just based on how much other junk was build up already.

I will pull the plugs out then and see, along with checking all the other stuff. Are those compression numbers something I should be concerned about (like needing an engine soon), even if they aren't causing this?

Generally, no. If you have a really fast starter, then those are kind of middle-of-the-road. A normal starter or even slow starter and that is a great engine.

My starter isn't slow, but my car is an '04 and I would bet that it has the original starter. Also, the engine is a mazda reman with 40k, so those seems pretty high compared to some of the stories I've tea about mazda reman quality

Thanks for everything :)

Yup, looks like you got a good one then.

I checked the intake valves. I don't know when they're supposed to open, but the VDI opened just past ~7k and the SSV opens just past ~6k. I'm gonna pull the plugs in the morning.

VDI is 7250

APV is 6250

I think your issue is with the SSV, have you tried moving it manually, does it move freely?