89 350 TBI problem

Discussion in 'Troubleshooting & Diagnosis' started by Chuck78, Jan 6, 2007.

  1. Chuck78

    Chuck78 Veteran Member

    Oct 22, 2000
    Columbus, OH, USA
    when it rains, it pours... 700R4 is failing, and just as I track down a used one, engine starts running poorly... The thing had a minor glitch for a year or two, where at 4 or 6 mph, light throttle, going up an incline, the rpm's would jump up and down at 2 or 3 second intervals. Now this new problem seems to be similar but 100 times worse... at low rpm's , the thing barely runs, and backfires through TBI sometimes. Once it hits 25-30 it smooths out, and by 35, it picks up power again. Trans is shot, but I think this engine problem is unrelated. It's in an 89 Suburban.

    I had broken a wire loom for the plug wires when I put a new set on, and I saw a wire that touched the exhaust manifold and burnt the insulation a little, but I put an old one back on and still same problem.

    Any clues where to start looking? I lost my Haynes manual or else I would have started troubleshooting it already. It is really hard to take off from a stop until I get it up to speed.
    New fuel filter a month ago improved performance a ton. I am wondering if it is in the fuel pump, sensor/computer or timing related issue, or what? I am no ace mechanic, but once I get on the right track I can figure things out.

  2. m pozzi

    m pozzi Moderator Staff Member

    HI Chuck,

    Any "Check Engine" light coming on??

    If not, you're looking for something non-electronic (most likely but not always) that's a non-codable fault. Check your basic adjustments such as timing, vacuum leaks (and these can be anywhere), wiring, and such. Another basic adjustment is the backside of the throttle plate as if there's gunk buildup on it, you've got minimal laminar air flow. In essence, the incoming air tumbles at this point and disrupts your air:fuel mixture at idle and off-idle. Once rpm and speed increase, the incoming air smooths out. Often, you'll notice an initial hesitation on low-speed throttle application as well. This is a simple fix and if the backside of the throttle plate has never been cleaned, start there first.

    Regarding the vacuum leaks, check the entire length of the intake ducting. From the air flow meter to the throttle body unit. Every inch!! On a vehicle that old, there has to be some brittleness in the hoses and you'll get air leaking past. This is unmeasured air and will play havoc on your O2 sensor's and ECM's attempt to keep that 14.7:1 AF mixture. To check for system air leaks, use a spray bottle of soapy water. Start the engine and warm to operating temperature, then spray away. If it's a small leak, the engine rpm will usually smooth out and then go back to it's poor idle. A more serious leak might be harder to locate with this method.

    Is the O2 sensor still alive?? There are replacement intervals for this sensor and I think it's around 90K or so. If the sensor's bad, that will also cause driveability problems but not the one(s) you're describing.

    I'm presuming the plugs and wires are good??

    Normally, a bad fuel pump will do the reverse of what you're describing. You'll get a decent idle but fuel delivery will be poor at higher vehicle speeds and especially under load. Also, most bad electric pumps are quite noisy and can be heard from outside the car. The usual life of a fuel pump is about 80-100K but not always.

    Regarding the 700R4 transmissions, they're POS certified and my recommendation for the replacement is never drive it in OD (fourth gear) unless you're on the freeway or at cruising speeds. Keep it in third for in-town driving or when towing anything. That's the kiss of death for these transmissions. The transmission pump is the weak link and if I remember correctly, the older units had a plastic sleeve or guide that would even melt under high temperatures. Not sure if the updates fixed this as I'm not a trans specialist.

    Good luck,
    Mary Pozzi
  3. Chuck78

    Chuck78 Veteran Member

    Oct 22, 2000
    Columbus, OH, USA
    The check engine light comes on after a minute or so, but resets when I restart it after it stalls, and then comes back on in a minute.

    I'll check that throttle plate, and any vacuum leaks.

    Air flow meter? Where is that in my setup? I thought the air was just ducted to the air cleaner just like a carb setup with this 89 TBI?

    No clue if the O2 sensor was ever replaced or not. Located in the exhaust manifold?
  4. m pozzi

    m pozzi Moderator Staff Member

    Your O2 sensor is in the exhaust header pipe below the flange to the manifold. I think it's on the right bank. You'll need an analog voltmeter to test it in closed loop. A high mV reading indicates a rich mixture and low mV indicates lean. The needle should fluctuate as the ECM is trying to meet the stoichiometric (perfect) mixture of 14.7:1. In essence, the O2 sensor is playing "catch up" with the system. It sends the mV signal to the ECM and that responds by adjusting fuel delivery. Also, any misfire or incomplete combustion will usually drive the O2 reading lean, not rich, and that mV will be sent back to the ECM which will, in turn, try to deliver even more fuel. The O2 sensor reads oxygen and since engine misfire leaves a lot of oxygen (read: unburned mixture) present, the exhaust leaves the combustion chamber and this surplus of O2 is "read" by the O2 sensor. You can pour fuel past the O2 sensor and it won't care a whit (for a while, that is). Oxygen is all it cares about.

    Can't remember if the TBI systems use a full air flow meter but if they do, it's round, oblong, and has a wire connector attached to it. There has to be something that lets the ECM know how much air is coming in to the engine so it can deliver the appropriate amount of fuel. Any air leak past the air flow meter isn't seen electrically by the ECM and is unmeasured air.

    Any chance you can have the codes pulled?? That "Check Engine" light coming on after a minute tells me the system reaches closed loop so it's not an open or short in the coolant temp sensor or throttle position sensor (both of these can produce your symptoms in addition to others you don't have). Your Suburban's system's not OBD-II so your code(s) are held in memory for 50 key ignition cycles from the last "Check Engine" light illumination.

    Once you get the codes and decipher their origin, don't just start replacing sensors and/or actuators but look beyond the box to figure out why the code is present. Sometimes replacing a sensor/actuator solves the problem but other times the root cause might be hidden and more difficult to locate.

    If you can get away with keeping repair expenses to a minimum, do it. However, if you start spending lots of time and are looking at a couple hundred dollars in parts . . . get the SUV to a good shop and pay for diagnosis and the repair. You might be $$$ ahead.

    Cyberspace diagnosis is difficult and good luck with this one.

    Mary Pozzi
    Last edited: Jan 6, 2007
  5. spoonLT1355

    spoonLT1355 Veteran Member Lifetime Gold Member

    Mar 1, 2003
    Rockledge Pa
    The 89 SHOULD have a MAF, they switched over to the MAP in 90.
    You could also have a bad TPS.

    Otherwise, Listen to Mary /\ /\
  6. Knuckle Dragger

    Knuckle Dragger Administrator Staff Member Lifetime Gold Member

    Nov 2, 2002
    Waddell AZ
    It's coming at me as a ignition miss. These things were really hard on wires. My first step would to do a really good visual inspection, cap rotor wires, plugs. Ohm out the wires etc.
  7. jakeshoe

    jakeshoe Veteran Member Lifetime Gold Member

    Dec 8, 2000
    Republic of Texas
    I've never seen a GM TBI 350 or 305 with a MAF, they all had MAP sensors, it was always a speed density system.
    Bad TPS is a good suggestion though, and those motors had issues with the distributor pickup coil.
  8. Fast77

    Fast77 New Member

    Mar 21, 2006
    Check you're fuel pressure... Sound like the fuel pump on it's last legs... I've got a '90 Blazer did the same thing to me about 2 years ago..
  9. spoonLT1355

    spoonLT1355 Veteran Member Lifetime Gold Member

    Mar 1, 2003
    Rockledge Pa
    My fault. I read it as TPI :confused: :confused:
  10. K5JMP

    K5JMP Veteran Member Lifetime Gold Member

    Hi, I have a 1988 Blazer. You are describing the same problem I had... I think:confused: .. Mine drove sluggish and all the obvious stuff looked OK. Is it very hard to start sometimes? Seems to stall for no reason and no pattern? If so I suggest this...
    Check your fuel pressure and flow. The pumps on those quit a lot... to the point I have a hatch in my Blazer:rolleyes: They don't flow enough thru the return to keep the volume up and thereby the pump cool... And just because you hear the pump pressurize the line doesn't mean it will re-start. It drove me ga-ga until I figured it out. Your clogged filter (the one you replaced) didn't help the life of your pump. You gotta keep-up with filters on these.. without correct fuel pressure the computer overcompensates and the injectors go into "drool" mode. In this mode the mixture is all over the place with the computer chasing the timing as well... not very pretty.. LOL!

    The other issue is, once again, gas additives. It attacks the plating on the wiring connectors. High resistance causes increased current and overheating of the pump. This is not a fire hazard that I am aware of... please see my disclaimer:rolleyes:

    IF you wind-up putting a pump in it, be sure you get the one with the replacement harness (3 flippin wires..lol!) but it doesn't have the reactive plating.

    As Jake says the TPS sensor is also a likely candidate... but kind of a pain to replace. I think you gotta dis-mount the injector body and it goes in from the bottom??:confused:

    I have also been told by "Mr. Goodwrench" that the magnetic pole-piece on the distributor shaft loses magnetism over time. This loss of magnetism will cause the electronic equivelant of "spark-scatter", resulting in un-even idle and stalling as you creep through parking lots.

    I hope this helps..:) YMMV!

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