engine hesitates, throttle response slow, power down under 4k

[General Geoff]

I suspect power is down a bit above 4k as well, but not as noticeably, and the hesitation seems to go away above 4k as well. I've already replaced the fuel filter, and the fuel pump only has a few thousand miles on it.

I checked my plug wells, they are pretty much dry (a little bit of dirt but no appreciable amount of oil); the manifold gaskets and valve cover seals and oil plug well seals were replaced less than 10,000 miles ago. Pic of one of the plug wells.

The engine does not throw any codes, and the car is running VERY reliably (just drove a thousand miles on a trip to North Carolina and back), but it is definitely down on power and throttle response. And the hesitation is also noticeable.

My next thought is vacuum leak somewhere, what do you guys think?


edit; car is a '93 MTX, engine has about 90k on it.

 

[Irish Pride]

Plug wires? Spark plugs?

 

[LJRuddy]

Any stalling out or rough starts?

 

[General Geoff]

Engine idles very smoothly and at proper rpm. No stalling, no rough starts. Plugs and wires were replaced about 10k miles ago.

 

[Irish Pride]

Unplug the MAF and see if it clears up. Drive with it unplugged.

 

[General Geoff]

I'll give that a whirl and report back, thanks for the suggestion!

 

[General Geoff]

Symptoms get even worse with MAF disconnected. idle gets lopey too.

 

[jimtash]

Consider replacing the throttle position sensor. Sometimes they go bad and don't throw a code but do produce a noticeable drop in power.

 

[General Geoff]

I ordered up all new vacuum hoses and intake couplers, which I should really do anyway since they're old and cracked. I'll try the throttle position sensor as well, worse case I just return it if it doesn't help since the local autozone stocks it.

 

[General Geoff]

Replaced all the couplers and vacuum hoses, for the first 10 minutes or so the car *seemed* to run a little bit better though still with hesitation and poor throttle response.

Then I got on the highway, and after about 10 miles the car seemed to run VERY poorly, as in, the car barely accelerated at all under WOT. Noise increased but very little additional power with extra throttle, this time even above 4k. So I'm thinking the vacuum lines and couplers aren't the issue. Next step: DIS and coil pack? Still no codes.

 

[SHOdded]

Jimtash had mentioned the TPS. Did you end up replacing that also?

 

[General Geoff]

I was told by my mechanic that replacing the TPS is a waste of time, but since it's so cheap, I'll give that a whirl too. If it doesn't help, I can just return it to autozone since it's a regularly stocked item for them around here.

 

[GR8WYT]

Had the same experience recently. Eliminated the other sensors, connectors, vacuum leaks etc. All that was left was the DIS. We replaced that, and all was well.

 

[RonPorter]

No power below 4K is classic inoperative secondaries. They default open, vacuum closes them when running until 4K. Simple thing to check.

Although, when mine did that, changing all the vacuum parts didn't help. Turned out to be a FUBARed PCM.

 

[General Geoff]

No power below 4K is classic inoperative secondaries. They default open, vacuum closes them when running until 4K. Simple thing to check.

Vacuum secondaries definitely work, first thing we checked after putting the intake manifold back on.

 

[General Geoff]

replaced DIS, acting the same. I guess the next step is coil pack, I just can't afford to keep buying new parts and not fixing the problem. Anyone in the eastern PA area (near Allentown) with a running SHO willing to lend me some of their time and parts temporarily?

 

[BoogiesAdda]

ndsbate But

I had the same problem and it was a combination of a vacuum leak and the TPS. The vacuum repair improved it but once I put the new TPS on it ran like it should. Make sure you bring AutoZone all the numbers and letters off of the OEM TPS there are like 4 different ones. I learned the hard way it was the last two letters that made the difference on mine

 

[SHOdded]

Coil packs rarely fail on the SHOs, though I suppose if they have coolant leaking on them, it is possible. Usually it's the firing order that's mixed up that is a problem in that area. You could check that again. Also, did you get a chance to swap out the TPS?

 

[LOUDSHO92]

What wire pattern did you use? If you get it wrong it can cause problems.

Also get the big hose in the back?

 

[General Geoff]

The firing order hasn't changed, I have not swapped the plug wires in any way since it was running well. When I checked the plug wells for oil, I was sure to pull one plug boot only, and replace it before proceeding on to the next.

Also, what big hose in the back?