Miata very slugish at low RPM, what things to check?

My 1990 Miata has become increasing sluggish at low RPM to the point where it is painfully slow pulling away from a stop. I've changed the plugs, plug wires and O2 sensor so far, but I guess I need to dig a bit deeper.

What are the next things that I should check? I don't have a timing light, but I suspect that might be something to check next. I've also read about throttle bodies becoming dirty, so I may need to check that too. I checked the compression a couple months ago and the numbers seemed okay (between 140 and 150 psi). Thanks in advance!

Cam timing?

Don't clean the throttle body, you'll end up with a high idle. There's some black sealant in there that the internet loves to think is dirt.

Cam timing isn't a gradual change. I'd be checking for things like a clogged up cat and fuel pressure. Also, check for stored codes.

Does it have the short nose crank? If so the timing keyway may be damaged. I had the same symptoms and problem on my 90. In my case it did come on gradually. You can check for this by adjusting the cam position sensor to the right or left (I can't remember which way) on the passenger side of the engine near the firewall. Then take it for a drive, If you regain power then the keyway is likely the problem. If it is you usually need a new crank, but I opted to have the cam gear welded to the crank that was fast cheap and got me another 5 years out of the engine.

Melted cat. It may have filled with fuel and burned. Pull it off and look in both ends. Check for melted undercoating in the area.

Woody wrote: Melted cat. It may have filled with fuel and burned. Pull it off and look in both ends. Check for melted undercoating in the area.

I would expect the symptom of that to be sluggish at the high end, not the low.

My first thought is the MAF sensor.

Short nose crank?

My 97 with the same issue became marginally better when the MAF was cleaned but the problem didn't completely go away.

EvanB wrote: Short nose crank?

Short nose crank

To add to my above post, the power loss happened so gradually as the keyway wore out, that I thought it was in my head. It wasn't until I was beaten in my local club's once a month auto-x by a car that I always handily beat, that I knew something was up.

Is a 1.6 liter Miata engine ever NOT sluggish at low rpm?

I had that once in my Mustang, then I selected the gear to the far left and up. Ran great after that.

Thanks for all of your feedback. It does have the short nose crank, so I will try adjusting the cam position sensor to see if I can diagnose whether it has a worn crankshaft keyway that has put the timing off. If that turns out to be the case, which of these options would you do:

Option 1: Try to repair the keyway on this engine (it has 275,000 kms on it)

Option 2: There is a 1994 1.8 Miata engine on Craigslist for $350. Ad says that the 4th piston is shot, but no damage to the block, and comes with a replacement OEM piston. Unknown mileage.

Option 3: There is a 1991 long nose crank 1.6 engine on Craigslist for $1200. Ad says that it may have had head work done because compression is 200 psi on all cylinder. Engine has 145,000 km.

Option 4: There is a 1997 1.8 engine on Craigslist for $1500. Ad says that the car was written off and includes a picture of the odometer at 189,000 kms.

Decisions, decisions.

Wow I can buy whole cars here for less than those engines.

If you're going to the trouble of replacing the motor and don't have to deal with smog-related issues, put in a 1.8. It's just better than 1.6 in every way.

That said, $1500 for a 120K mile '97 motor sounds like a lot. Personally I'd look for a 99-00 motor or 01-05 if you're willing to deal with the complexity of managing the VVT system.

With that kind of mileage a compression/leak down test would be a good idea.

Worn rings or a worn camshaft are possibilities

Have you plugged a code reader into it? Those seem like pricey motors.

He's in Canada. There aren't as many junkyard Miatas as there are in the US. Of the above options, I'd go either with Option 3 (best engine, easiest install) or Option 4.

Also, 1990 Miata: no MAF, no code reader. However, you can read codes off the CEL by using a paperclip. I'd start with that.

For the keyway check, another option is to check the position of the timing mark when the engine is at TDC. Set TDC using a dipstick or dowel down the #1 plug hole, then the timing mark should be at T. If it's not, you either have a keyway problem or you have a failing damper It's true that a failing crank nose would be a gradual change, I assumed "cam timing" meant a slipped belt, which is more of a legend than an actual thing.

Another option: at 275k km, you're only 25k km short of a timing belt change. Pull the crank pulley off and inspect the crank nose.

The 1.6 engine is actually more fun to drive than the 1.8. Not as fast, but it's got a more entertaining cam.

In reply to mainlandboy: If it is indeed the keyway, rather than changing the engine you should try out the weld timing gear to the crank fix first, although before doing it you should change the oil seal.

It is a inexpensive and fast fix. For me it turned out to be a permanent fix. Just make sure to line up the timing gear correctly to the keyway.

My local Mazda shop did this for me and at the time it cost less than $200.

Keep in mind that you'll never change that front main seal again. A preferred method is the Loctite fix, which allows future service. Plus it's easy to do at home.

We've got a customer with over 400k miles on his 1.6, so don't assume you'll never want to get at the seal in the future!

In reply to Keith Tanner:

Good point!

Thanks for all of the feedback! I'm going to check the crank pulley alignment mark against top dead center to see if the keyway is off. I found the Loctite fix instructions here:

http://www.miata.net/garage/hsue/crank/loctite_1.htm

I found a very good video tutorial here:

https://www.youtube.com/watch?v=dMhKGb_Duzs

I noticed in the video that he used JB Weld instead of the Loctite 660 that was listed in the instructions. Do you think that one might be better than the other?

The Loctite fix was figured out by engineers at Loctite and is proven. I'd be hesitant to take the choice of a random Youtuber instead. JB Weld is a lot easier to find at Home Depot, which is probably why it was chosen.

Loctite works by expanding in the joint.

I doubt JB weld is anything more than an epoxy.

Loctite, the company, manufactures a lot of different compounds. The Loctite fix for the crank isn't just blue thread locker on the bolt (that's actually SOP), but a few different types used in different ways. One is called liquid metal, it's not thread locker. But it's also not a two part epoxy.

Even epoxies vary. Just read up on the strength differences between JB Weld and JB Quik. JB Weld is actually a pretty good one if you can keep it from drooling before it sets.

Well, it looks like I've got the short nose crank keyway wear issue. I pulled the #1 plug and dropped the dipstick down the hole. I then rotated the engine until piston 1 was at TDC. I then put a Sharpie mark on the crank pulley where the "T" was on the timing cover. I then rotated the engine a bit more to see the offset between the Sharpie mark and the notch in the crank pulley. Here is the result:

It looks like I'll be attempting the Loctite fix.

In the meantime, since the camshaft spins at half the speed of the crankshaft, is it safe for me to temporarily correct the ignition timing by rotating the cam angle sensor by 1/2 the angular offset that the crank is off by, as recommended by drsmooth above?

It could be the crank issue, it could also be a failed damper. They're two pieces of steel bonded to rubber, and when that bond fails they slip relative to each other.

Still, you're almost due for a timing belt, so pulling that front assembly apart to inspect isn't a terrible thing regardless. Looks like a seal or two in the front end wouldn't go astray either

Your plan will correct the ignition timing, but it won't correct a valve timing problem.