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'90 Firebird 3.1L perfect when cold, misses when hot, very rough

Tiny
THADIUS856
  • MEMBER
  • 1990 PONTIAC FIREBIRD
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 106,000 MILES
Brought my '90 V6 cross country. Went to get a CA smog, it failed on 2500 rpm CO. Immediately went to DMV to get a temp tag, and it barely made it back.

Now when the engine hits 160* or 180*, it immediately loses all power and begins missing. Like throwing a light switch. One second it's fine, the next it's running extremely poorly. If you depress the accelerator more than a hair, it stumbles and will backfire if you press hard enough. It has just enough power to hit 20 mph on perfectly flat ground after 30 seconds of acceleration.

I work on a military base, and it's at the shop there. It runs great cold, but there's no way it would make it the 15-20 miles into town.

I started with a set of plugs. New wires. New cap and rotor. Other than a slight oil leak on one valve cover, all looked great. Injector cleaner in tank. Fuel treatment. Tank is full. New oil, new air filter, new fuel filter.

Mechanic found a hole in the radiator, thought that air cavity was causing temp misreading and ECM was dumping fuel to warn engine. Replaced, pressure tested all hoses good. Mechanic tossed on new EGR (I think?), But says it had no effect so took it back off. Replaced factory O2 sensor (oops!). Said IAC looked brand new and clean.

He's out of ideas. I'm almost positive it's not fuel flow. Fuel pump less than 1 year old / 5k mi. Have not had time to check pressure at rail, but the backfiring makes me doubt it.

Only out $290 for radiator and O2, including labor. Plus cost of tuneup parts.

Need ideas for things to troubleshoot. Have full access to bays, lifts, most of the shops tools. Can get free mechanic assistance. Or hire mechanics at reduced hourly rate. But not idea where to go now.
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Thursday, May 26th, 2011 AT 10:55 PM

22 Replies

Tiny
FIXITMR
  • MECHANIC
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Definately get a pressure test. Also MAF sensor try unplugging it.
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Thursday, May 26th, 2011 AT 11:26 PM
Tiny
THADIUS856
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Will attempt both right after work on Tuesday. Where can I find the MAF sensor connector on this vehicle? It's not throwing a Check Engine light when it acts up.
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Saturday, May 28th, 2011 AT 4:55 PM
Tiny
FIXITMR
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Maf should be on air intake hosing.
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Saturday, May 28th, 2011 AT 8:17 PM
Tiny
THADIUS856
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Mechanic took a look at fuel pressure. Said it looked good both when off, when cold (running great) and when hot (running rough).

Also said that MAF isn't installed on this vehicle. Typically not until about the same time that OBD2 rolled out.

More ideas? Seems more electrical to me from how suddenly it acts up. :
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Wednesday, June 1st, 2011 AT 9:42 PM
Tiny
THADIUS856
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The shop still hasn't come up with any ideas. Bump.
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Thursday, June 2nd, 2011 AT 11:11 PM
Tiny
FIXITMR
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They sell a maf so if you dont have that you must have a MAP? Also what about ignition module? You can get it tested. Also distributor hall effect coil could be bad.
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Friday, June 3rd, 2011 AT 5:27 PM
Tiny
THADIUS856
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Just moved into a new house this month. Been crazy busy, sorry I haven't posted an update recently. Still hasn't been fixed.

Had the vehicle towed to a new shop at the first mechanic's recommendation, this time a Gold Shield location. Technician at new repair facility found that by disconnecting the alternator lead and its harness that the car would run roughly, but at least stay running for some time. He blamed the issues on internal arcing (or was it internal short?) Of the alternator. He also cited high resistance readings on the injectors as the reason for failing smog. I towed it home, expecting to not fix it for several months. But now I'm ready to dig in.

Pulled alternator for bench test at a Napa. Passed good @ 14V. Brought it home, reinstalled, still won't run when plugged in. Still runs rough when unplugged from terminal and harness. Pulled battery, tested bad @ 6V. Parts store guy thinks it got fried. Replaced with new battery. No change in symptoms.

Pulled alternator again, had it tested twice at another parts store. Tested OK twice @ 14V. Brought it home, reinstalled it.

Without throwing $70 at the chance of the alternator tests being flawed, what are my options? I'll be doing the diagnostic measurements from now on, so I'll pick up a voltmeter if need be.
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Tuesday, August 2nd, 2011 AT 1:25 AM
Tiny
THADIUS856
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Bump.

I'm disconnecting every electrical connection I can find and cleaning them, one at a time. So far, no changes. The vehicle starts and runs for a few seconds, then dies abruptly. Each time I attempt to start it, it runs for a slightly shorter period of time until I let it sit for a little while.

I found a disconnected vacuum line. It is not on the 3.1L EFI vacuum diagrams I'm using on AutoZone's website. I've found and checked all of the connections on that diagram, except the "to PCV valve" leg, as I'm not sure where I'm looking.

The section I found disconnected runs from nearly under the battery, up the driver's side of the engine compartment, across the aft section near the brake boost, and lies disconnected near the rear of the block. The line is fairly small, with maybe a 3/8" outside diameter. I'll try to add a picture of it.
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Sunday, August 7th, 2011 AT 4:03 PM
Tiny
THADIUS856
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Attached the vacuum diagram I've been using. As above, all legs look good, except for the PCV/Inlet Manifold leg that I don't know where to look for. Notice that there are more vacuum lines coming off the rear of the block that aren't in the diagram.

The vacuum line held in the second image is the one I found disconnected running from all the way down by the battery, and can't locate its home. It doesn't look cleanly cut.
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Sunday, August 7th, 2011 AT 4:10 PM
Tiny
FIXITMR
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2 things have a dramatic effect on mixture control computer temp sensor and map/maf sensor. The other possiblity is vacuum leak? Most backfiring is either timing or lean condition. So it dont run good at all anymore? Used to wait until it got hot to screw up? Is the computer coding ok? By the way, i'm workin on a mystery problem with an old car of mine. Did intake gasket and now it wont run right/idle. I think the vacuum lines are messed. It's a 85 2.8 v6 which came out before yours. Luck to both of us!
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Tuesday, August 9th, 2011 AT 2:42 AM
Tiny
THADIUS856
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Computer is coding fine. Was swapped by first mechanic with no effect. Does not idle anymore. Will crank fine, but will not idle. Timing was "spot on" when I had it smogged - I don't think he had to disconnect anything to check it.

I assume temp sensor is fine, because it shows accurate readings on the cluster (assuming they use the same sensor).

MAP too expensive to just swap in. How do I test it?
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Friday, August 12th, 2011 AT 2:05 AM
Tiny
FIXITMR
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Try disconnecting MAP and see what changes? See if CEL correctly identifies error? You should have seperate temp sensors the 1 for puter is usually around thermostat housing. Mine is left of and on top. My 85 has a MAF sensor and it's original and never been cleaned so I may check it to see if it got dirty from layin around.
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Friday, August 12th, 2011 AT 3:36 AM
Tiny
THADIUS856
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Swapped alternators to no effect. Sprayed WD-40 into the intake and it runs fine until the supply is exhausted. Purrs even! Now not so trusting of the mechanic's fuel pressure reading. I think my next step is to check pressure at the Shraeder valve just before the pressure regulator.

Couldn't find the MAP. Taking a few minute break to see if I can make my eyes fresh before I go back out and look again.

I've never checked pressure here. Any specific tips for testing? Any specific type of pressure tester I need to pick up? What is the range of acceptable pressures at this location under what conditions?

Thanks!
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Friday, August 12th, 2011 AT 4:09 PM
Tiny
FIXITMR
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Key on 60+ RUNNING 55-57 is what I recall.
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Saturday, August 13th, 2011 AT 3:19 AM
Tiny
THADIUS856
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Been busy with electrical on the home this weekend, so I haven't gotten to the pressure test yet. Will post when I get that taken care of.
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Monday, August 15th, 2011 AT 4:46 PM
Tiny
THADIUS856
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As a side note, located the MAP. Disconnected it and it lit up the check engine light. Didn't seem to have any effect starting with it disconnected, so it was promptly reconnected.

The other set of hands on the car decided to replace the oil pressure sender thinking it might be sending low oil pressure to the ECM, which could cause it to cut fuel pressure drastically. I think it's a step in the right direction, but unfortunately it didn't solve anything, and is the kind of throwing money at replacing parts that I despise.

Will keep you posted!
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Monday, August 15th, 2011 AT 4:48 PM
Tiny
FIXITMR
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Sorry, somehow i'm getting this and another topic mixed up! Fuel pressure with key on not running should be 40+. While running should be slightly less like 35-39.
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Monday, August 15th, 2011 AT 5:50 PM
Tiny
THADIUS856
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Checked vacuum hose at fuel pressure regulator again. Don't smell fuel this time after cranking several times. I think I may have just been taking a whiff of the WD-40 that was on the fingers I was using to hold the hose.

Purchased an El Cheapo fuel pressure tester at Harbor Freight ($20 - 20% Sunday paper coupon = $16+tax) that came with adequate attachments, since no parts store seems to have their rental tester in stock. After a little fuel squirting the block because I forgot to use Teflon tape (woops, fixed that PDQ), we got a reading.

39-41 psi key-on
38-45 psi cranking and (attempting to) idle
29 seconds to drop from 40 psi to 20 psi after removing key.
Readings were consistent every time

Looks like we can count out the fuel pump, relay, and filter as a cause. Everything aft of the Schrader valve on the rail must be OK.

Again, I think the fact that I can remove the air intake, spray in WD-40 and get 5-10 seconds of perfectly smooth idle to be a pretty big clue.

Will pick up a multimeter in the morning, just in case.

Ideas?
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Monday, August 22nd, 2011 AT 11:05 PM
Tiny
FIXITMR
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I remember a problem that turned out to be a plugged sock on a fuel pump. Showed the right pressure but had hardly any flow. You could disconnect a line and check flow? Not enough fuel would cause your symptoms.
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Tuesday, August 23rd, 2011 AT 3:02 PM
Tiny
THADIUS856
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Checked fuel flow yesterday when the gauge tried to squirt me with fuel. Plenty of it.

Have an over-the-top access to the fuel pump I created a year back, but it's still a real bear to disconnect the lines to check the strainer. New strainer last year when I put in a new fuel pump, that replaced the original set.

Will go reconnect to the test hose to the Schrader valve, without gauge, and see how much comes out into a cup with key on accessory.
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Tuesday, August 23rd, 2011 AT 3:07 PM

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