Bad mileage + hesitation

Fastback86 · Post by **Fastback86** » Sat Mar 22, 2008 1:00 pm

So all of a sudden, my gas mileage dropped noticeably. I usually swing about 170 miles on a tank, but the last few tanks its been more like 140, and that's scraping the bottom. I didn't give it too much thought since everybody gets shitty mileage now and then depending on driving conditions.

However, recently I've noticed what feels like a miss. Odd thing is, I only feel it while cruising on surface streets. I really haven't noticed it on the highway, and I don't feel it under acceleration. At first I thought it was all in my head, but now I'm starting to wonder if these things are related.

I haven't worked on the engine recently. The distributor was rebuilt fairly recently and the plug wires are fairly new. The plugs themselves should have plenty of life left on them as well. Unfortunately, I don't have a scan tool, so all I can tell you is that it isn't throwing codes. My alternator belt is a little loose and squeals when it's cold, so I do need to tighten that up. I'm wondering if my A/C Compressor is starting to seize, it's been running for a while without anything but air and oil in the system, but it should be spinning freely since I haven't engaged it at all.

Any thoughts on where to start? I looked around and didn't find anything obvious (no spark plug wires on exhaust manifolds or anything).

whipped · Post by **whipped** » Sat Mar 22, 2008 1:17 pm

V6?

15mpg is terrible unless you're driving like andretti. Could be a leaking cold start injector or FPR. It's been a while so I don't know how to check those on a stock fiero.

Another thing that's easy to check is timing. If it slipped you could run into those kinds of problems.

The Dark Side of Will · Sat Mar 22, 2008 2:14 pm

How much did you pay for your plug wires and how long have they been on the car?

Fastback86 · Post by **Fastback86** » Sat Mar 22, 2008 2:15 pm

Ya, 3.4 pushrod. I'll check the timing as a precaution. I've been thinking about blocking off the Cold Start Injector, maybe I should just do it and be done with it. I need to get a fuel pressure tester so I can see, but I'm reasonably sure its not the fuel pump. I dunno how you check the regulator, or that they could go bad. I thought it was just a vacuum operated diaphram. Also, I can't check my plugs right now cause I looked in my tool box and apparently, my spark plug socket was a cheap piece of shit, cause it broke at the U joint just sitting in my tool box. Lame.

Right now, my top suspects are the Cold Start Injector and the plug wires, cause last time I had a hesitation like this it was a bad wire that was broken inside but looked fine on the outside.

Fastback86 · Post by **Fastback86** » Sat Mar 22, 2008 2:18 pm

The Dark Side of Will wrote:How much did you pay for your plug wires and how long have they been on the car?

They're AC Delcos and I don't recall what the cost. They've been on maybe a year or two, I don't recall exactly. Can't be more than 10,000 - 20,000 miles at the absolute most.

The Dark Side of Will · Sat Mar 22, 2008 2:35 pm

I had a $20 set of Advance Auto plug wires cause my car to start to exhibit those symptoms within 6 months of installation.

Fastback86 · Post by **Fastback86** » Sat Mar 22, 2008 2:41 pm

Good to know, maybe it's just time for a tune up. I wish there was an easy way to tell if plug wires are good or bad. Ah well, I'll just suck it up.

whipped · Post by **whipped** » Sat Mar 22, 2008 2:59 pm

Fastback86 wrote:I dunno how you check the regulator, or that they could go bad. I thought it was just a vacuum operated diaphram.

Exactly. And sometimes they like to spring a leak. Check the vacuum nipple for fuel.

Fastback86 wrote:Good to know, maybe it's just time for a tune up. I wish there was an easy way to tell if plug wires are good or bad. Ah well, I'll just suck it up.

There is.... Start the engine when it's dark and mist the wires with water. If you get a light show or hear any arcing, the wires are bad. Works for distributor caps too.

Blue Shift · Post by **Blue Shift** » Sat Mar 22, 2008 6:07 pm

Hey, I'll headed down to Steven's in not too long here - if you're in that area, we should all gather round. Plus we both have fuel pressure testers and other shit too. I gotta trace a problem with gas coming out somewhere - I can smell it when I get on it, but there aren't any apparent leaks in the engine bay.

Fastback86 · Post by **Fastback86** » Sun Mar 23, 2008 2:27 am

Blue Shift wrote:Hey, I'll headed down to Steven's in not too long here - if you're in that area, we should all gather round. Plus we both have fuel pressure testers and other shit too. I gotta trace a problem with gas coming out somewhere - I can smell it when I get on it, but there aren't any apparent leaks in the engine bay.

Use a lighter to get a better look at things :thumbleft:

I'm working for the next couple of days 2-10pm, then I'm leaving on vacation Wednesday through Saturday, but if you guys are in the Bay Area in the morning, let me know.

Fastback86 · Post by **Fastback86** » Sun Mar 23, 2008 2:29 am

whipped wrote:
Fastback86 wrote:I dunno how you check the regulator, or that they could go bad. I thought it was just a vacuum operated diaphram.
Exactly. And sometimes they like to spring a leak. Check the vacuum nipple for fuel.

Fastback86 wrote:Good to know, maybe it's just time for a tune up. I wish there was an easy way to tell if plug wires are good or bad. Ah well, I'll just suck it up.
There is.... Start the engine when it's dark and mist the wires with water. If you get a light show or hear any arcing, the wires are bad. Works for distributor caps too.

Good deal, I'll check it out. Thanks.

Fastback86 · Post by **Fastback86** » Wed Apr 02, 2008 7:53 pm

Ok, she's got a new set of plug wires now, the alternator belt is tightened (though still not quite enough, but I'll deal with it later) and the Cold Start Injector is unplugged. Predictably, it takes a little longer to start now, but I don't have any problem living with that. The engine was not exhibiting any noticeable symptoms at idle before, so I can't say whether the problems been solved yet, we'll see when I take it for a drive and the next time I fill up.

For now though, I do have one question. When revving it in neutral, I noticed that it is smooth coming off idle most of the time, whether I'm slow and gentle with the gas pedal or give it a light jab. Revs up fast and smooth. However, I noticed that if I stabbed the pedal hard, there's a little hesitation before it revs up. I honestly can't recall if it always did that or if that's new/a sign of my problem not being solved. The engine was cold when I was doing this (not revving over 4k, don't worry), so that may make a difference, or the fact that when pushing the gas pedal hard off idle, you're suddenly flooding the cylinders with a lot more fuel at low RPM rather than building it up more smoothly as it would when driving.

Am I making a mountain out of a mole here, or might this be a sign that I have not yet fixed the problem? I'll let you guys know after I've driven it a bit and see if it feels different and if my mileage improves.

The Dark Side of Will · Thu Apr 03, 2008 7:22 am

With you CSI unplugged, you're actually leaning out a bit when you stab the gas. That's what's causing the hesitation. It'll be fine once it warms up.

Fastback86 · Post by **Fastback86** » Thu Apr 03, 2008 1:22 pm

The Dark Side of Will wrote:With you CSI unplugged, you're actually leaning out a bit when you stab the gas. That's what's causing the hesitation. It'll be fine once it warms up.

Aha! I had a feeling it wasn't a problem, but I wanted to check. You know how it goes, you fix or upgrade one little thing on your car and as soon as you drive it, you're trying really hard to feel an improvement or a problem, especially if it's not an obvious one. The ol' butt dyno kicks into high gear and you start thinking that your new coil gave you 10 extra horsepower, or that every little bump in the road is a hesitation. I was trying not to get overzealous. For example, I drove it yesterday a little bit, and it felt a little pepier and smoother, but I don't know how much of that was my imagination. I'll just have to wait for the fill up and see how I do, probably be another week at least.

p8ntman442 · Post by **p8ntman442** » Thu Apr 03, 2008 7:55 pm

my car has the same bog on throttle tip in like your describing. I think its poor injector flow. If you think of it as a rc circuit, the poor flowing injectors are adding resistance to the system. This will slow the time it takes to reach steady state (where the TPS says it should be). I'm swapping in a set of flow tested injectors when I replace the tranny and clutch.

to back this up, my aldl scans show high blms across the board, which means leaky exhaust manifolds, or the injectors cant flow enough to satisfy the ecm's call for flow.

The entire ignition system is in good condition with magnecore wires, but the engine dosent have a timing tab, so its hard to confirm the timing is not the culprit.

Fastback86 · Post by **Fastback86** » Mon Apr 21, 2008 3:52 am

Well, the plug wires smoothed things out and took care of the hesitation, but I'm still getting shit gas mileage. My friend was driving behind me on the way to work the other day and remarked that it smelled like it was running rich. I'm starting to suspect a bad injector. I'm going to rule out the CSI since it takes noticeably longer to start now with the CSI unplugged, telling me that it is closed. I also suspect an injector because it's happened before. I had a dead cylinder a few years back because the injector had stuck closed. I replaced the bad injector, but left the rest, leading me to think another one may have locked up now, only open this time. I don't think it's a clogged fuel filter or anything since the problem came on very quickly. Maybe it's time to spring for those Mustang 19# injectors I never got.

Thoughts?

The Dark Side of Will · Mon Apr 21, 2008 7:43 am

Check how fast your fuel pressure bleeds down after priming. That's a lot easier to do than pulling the fuel rail.

Fastback86 · Post by **Fastback86** » Mon Apr 21, 2008 12:25 pm

True that. Guess it's finally time to get that fuel pressure tester.

Pyrthian · Post by **Pyrthian** » Mon Apr 21, 2008 12:34 pm

how's your temp gage reading? if it is at or below 1/4 - you are running to cold, and the ECM is not going into "closed loop mode" - which usually gets ya around 17MPG

Fastback86 · Post by **Fastback86** » Mon Apr 21, 2008 2:17 pm

Pyrthian wrote:how's your temp gage reading? if it is at or below 1/4 - you are running to cold, and the ECM is not going into "closed loop mode" - which usually gets ya around 17MPG

Actually, it usually does read around 1/4, but its been like that a long time. This came on pretty quick. I do have the low temp thermostat in there, I'll have to take a look at it and see if its stuck open or something.