Stabil vs car sitting periods

[mytrplseven]

I've been chasing some engine performance issues, i.e., stumbling on accel and uneven idle occasionally. I've chased a lot of the mechanical/electrical possibilities:
Vacuum leaks,
TPS
AFM
injectors
Bosch copper + plugs and new wires and distributor parts
New gaskets on intake manifold
Fuel pressure test for defective fuel pump and regulators/dampers (all ok)
New fuel pump and filter (tank screen was clean)
DME connectors and solder checks inside
Intake Filter
DME Temp sensor

Still, I have the above noted issues. Since the car sits for a month at a time due to other priorities (honey do list), I'm now wondering if maybe the gasoline might be the issue. I don't fill it up for around 3 months. The light went off last night when I was reading Gawvernators post about storing the car and thinking that perhaps Stabil may cure my problem.

Anyone have any sure-fire observations on this issue?

[Panzer909]

Stabil works to avoid the problem you already (potentially) might have.

I would first dump some chevron techron into the tank and then give it an Italian tune up by running it hard for an afternoon. See if it runs better.

But yes, I would try putting a little stabil based on your current schedule. Look at some of my last threads - all those running problems came from letting it sit too long.

[Gawernator]

I find that a cocktail of Techron, occasionally seafoam and then also the red bottle that Red Line makes works great for cleaning out crud. My intake valves are pretty much spotless as my testimony.

[samevansjr]

I use a can of Seafoam in every tank! I don't drive it a lot but when I do I know the gas is in good shape.

[mytrplseven]

I'm at 1/2 tank and a hard day out is a good way to purge out the older gas. A little techron and then some Stabil with a fresh tank is probably a good way to begin. It'll be interesting to see if that's been the answer all along.

[AaronM]

Gasoline stays just fine for 12+ months. I've never had any vehicle I've owned (currently two motorcycles, two Porsches, and one Volvo, with at least another half-dozen other cars of various make over the last 15 years) have problems from gas 12 months old or less.

Now, if you were leaving it alone for 12 months or more I'd say that you definitely want some Sta-Bil or something. But for only 3 months I just can't see it being worth the hassle.

[alan6272]

here in NC it takes 12 months for gas to turn, My car may sit for 3 or 4 months sometimes and I just make sure its full of gas when it does have to sit. and some stabil goes in then.

[mytrplseven]

I appreciate all the thoughts on this. I'm hoping for something simple as the cause of my problems. I guess I'll retrace the things I've done so far and see if there's something I may have missed. A little Techron or Seafoam (which I tried once last year) surely won't hurt. I want so badly for this engine issue to be history so I can really enjoy how good these cars are when the engine's right.

[gtroth]

Seems to be the right list.
What was done to the AFM and injectors?
What did the old plugs look like when you changed them (lean/rich)?

Coil, ref&speed sensors, compression/leakdown might also be considered I suppose.

[mytrplseven]

I rebuilt the AFM and cleaned the contacts on the TPS. Although I did read somewhere, that I may have wasted my time with the TPS and should have just replaced it. I ordered a new one yesterday and will start from there. The injectors were rebuilt by Witch Hunter in Washington and the electrical connectors were replaced as well, since one was suspect. New Beru wires and Bosch copper + plugs (ordered NGK plugs at same time as TPS just to compare) and cap and rotor. I also plan to read the plugs again when I change them but the last time they looked normal. Car seems to run fine but does occasionally stutter when returned to idle and stumbles on accel. Did a full vacuum test with air compressor and found 3 leaks, including the seals around the throttle body shaft. Pulled the Intake so I could clean the engine and get access to everything including the AOS seals and ISV. This just feels like a fuel/air issue. The Air cleaner was the first thing as it had a huge mouse nest inside from the PO. Drove the car from Santa Fe to SW Florida that way and even though the engine didn't run as it should have, still got 30 mpg at 70 mph.


Did a full fuel system test with gauges and even though I replaced the fuel pump, filter and check valve anyway, everything checked out. The screen strainer was clean too. I still feel I'm missing something and am hoping to find it before I have to spring for a MAF 944 NA (even though I'm convinced it's a great upgrade).

[AKCJ]

I agree with replacing the TPS (just due to age and lack of success by others rebuilding it).

I think you mentioned new plugs/wires/cap/rotor but how about the coil?

Probably wouldn't hurt to try a strong dose of seafoam or techron. I don't think stabil will "cure" the problem - it is to keep good gas good. Not make bad gas good.

Your description makes me think of AFM but you've already rebuilt that.

I would think you would want to solve this problem before switching to the MAF system.

Good luck and let us know.

[mytrplseven]

I replaced the coil as well. I also ran some Seafoam through the tank last year. I also didn't see anychanges when I poured some into the vacuum hose at the Power Brake Booster either, nothing happened. No smoke, nothing. I guess we'll see what happens after I replace the TPS and new NGK plugs. A little Stabil on occasion certainly won't hurt anything. I did rework the AFM but when I bought the car, the cover of the AFM had been removed before and if this doesn't solve the problem, I'll go back in an do a more detailed inspection and "fine tune" if necessary.

[gtroth]

Sure would be good to try a known good AFM if you can. Fishy that it had been opened, and will cause stumbling / hesitation issues if it lies to the DME.

[Panzer909]

It's funny. I did the Seafoam thing to mine before and absolutely nothing happened as well. Not even a puff of smoke from the exhaust. Which is strange because I did it in other cars, notably my E30 a few years ago. With that one, there was so much smoke I was concerned that someone in the neighborhood was going to call the fire dept. No joke. But I don't see how a 944 took it so differently (and I somehow doubt that it was because its engine was 100% clean with nothing to burn off).

[mytrplseven]

Groth: When I first looked into the AFM the cover had been removed. I moved the wipers, fixed a sticking barn door and cleaned all the connections.

Panzer: I'm wondering if these cars are immune to additives. Perhaps pouring the Seafoam into the hose to the Brake Booster wasn't the proper place.

Replaced the TPS with a shiny new Bosch unit and although I didn't have time for a proper road test, I did start the car and it idles at 900 rpm cold and seemed to rev quicker than before. Keeping my fingers crossed that I finally found the gremlin that's plagued my engine for so long. The old TPS appeared to have been tampered with because the plastic case was chipped.

I also ordered some NGK copper plugs and some indexing shims to properly locate the plug ground tip so it's functionally located to get a better flame front when the plug fires. There's some evidence that it actually does help perfomance a hair. Can't hurt, right?

[gtroth]

Originally Posted by mytrplseven

Groth: When I first looked into the AFM the cover had been removed. I moved the wipers, fixed a sticking barn door and cleaned all the connections.

...

Yup, all good things to do. Some hacks will adjust the spring tension to try and tune/debug, so be aware of that. That changes the door angle (reported air flow rate) vs actual airflow rate relationship. And the resistance vs door angle relationship might not be what is should be if it is worn in spots (ie might not be a smooth relationship). So a few subtle, hard to detect ways for the AFM to be messed up and tell the DME that the wrong amount of fuel is called for. Which is why for me it's always a prime suspect for these types of problems.

[Gawernator]

About the smoke. The first time I used it I got a lot of smoke. Last time almost none. Both through brake booster

[mytrplseven]

Well I finally took her out for a real street run after replacing the TPS with a new one. It went to red line so fast I couldn't believe it. It was like I had given it a "squirt". The curious part now is that the idle with the A/C off is around 1200 rpm and with the A/C on, it's 950. I need to figure out how to get them both down around 900. Before I replaced the switch, the rpm's were the same, regardless of the A/C mode. I'm open to input on this matter.

There's still a little hesitation that I'm thinking may be plugs or the AFM. I'll be putting my new NGK's in shortly (indexed) and then go from there.