Ongoing issue - every 35 seconds - slight bog/miss/whatever
01-28-2007, 05:40 PM
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Car: 1989 Firebird
Engine: 2.8L V6 MPFI
Transmission: Beat to heck 700R4
Ongoing issue - every 35 seconds - slight bog/miss/whatever
Hi all,
Been to hell and back on this one - 1989 2.8L firebird. MAF/MPFI, mostly stock (custom ****-warm air intake that takes air from near the canister where the old air filter box used to be - K&N air filter, TB heater bypassed, 180* stat)
When everything is plugged in the car starts up, idle is a little muddy but it otherwise works. Its great in open loop, but once it hits closed loop mode every 35 seconds - to the microsecond, its like the car skips a beat. Idle drops by 200RPM, motor sound goes down, car glubs for about 1/4 second, then comes right back like nothing happened.
Its as if I pinched the fuel line, or unplugged a vac line, or reached up under the EGR and pushed up the valve. 5% of the time this stumble stalls the car.
Otherwise it works well.
New MAF, intake is secure. Fuel pressure is 50+ after priming, stays at 50 for at least 5 minutes, and 43 at idle, and after turning the car off it was at 40.5 after 5 minutes. Rock solid fuel pressure. When the dip happens fuel pressure stays constant.
Vacuum via the brake booster is 17 inches. revving it drops the vacuum a few PSI as expected. When the dip happens vacuum dips with it. In other words, a component operated by vacuum (which may leak when actuated) isn't causing it as the vacuum dip follows the stumble, the stumble doesn't follow a drop in vacuum.
I've removed the cruise/reservoir/brake pedal vacuum line from the back of the check valve off the back of the plenum and plugged that part of the check valve, thus removing that portion of the vacuum system - same results. So one of those things isn't leaking.
I had been thinking EGR all along - I can actually *fix* it, by unplugging the MAF. Car actually runs a good bit *better* without the MAF plugged in. I'm assuming this is because unplugging the MAF cuts off some other parts of the emissions system?
Today I got to see for sure as I have a vacuum gauge. Its not the car actuating EGR - and it being a weak spring opening the valve too much as I monitored EGR vacuum when it occurred and no vacuum was present. EGR vacuum works, as I rev the motor and get about 1 inch of vacuum on the downside. So the EGR solenoid and switch are working.
Leaving EGR unplugged and the vac port plugged up results in a 32, that goes away when I clear the codes and plug it back in. So I believe the EGR itself isn't bad. I suppose it could be stuck open, but actuating the valve changes the motors idle, so I don't think so. I could be wrong here.
Only codes I've ever seen are 15 (I think I caused that by shorting the ECT with the TB heater bypass (metal tie-cables aren't good for electrical cables!). 33 and 34. I know 34 is from having the MAF unplugged. I've not seen the 33 in a while either. So essentially no codes.
Any thoughts?
Been to hell and back on this one - 1989 2.8L firebird. MAF/MPFI, mostly stock (custom ****-warm air intake that takes air from near the canister where the old air filter box used to be - K&N air filter, TB heater bypassed, 180* stat)
When everything is plugged in the car starts up, idle is a little muddy but it otherwise works. Its great in open loop, but once it hits closed loop mode every 35 seconds - to the microsecond, its like the car skips a beat. Idle drops by 200RPM, motor sound goes down, car glubs for about 1/4 second, then comes right back like nothing happened.
Its as if I pinched the fuel line, or unplugged a vac line, or reached up under the EGR and pushed up the valve. 5% of the time this stumble stalls the car.
Otherwise it works well.
New MAF, intake is secure. Fuel pressure is 50+ after priming, stays at 50 for at least 5 minutes, and 43 at idle, and after turning the car off it was at 40.5 after 5 minutes. Rock solid fuel pressure. When the dip happens fuel pressure stays constant.
Vacuum via the brake booster is 17 inches. revving it drops the vacuum a few PSI as expected. When the dip happens vacuum dips with it. In other words, a component operated by vacuum (which may leak when actuated) isn't causing it as the vacuum dip follows the stumble, the stumble doesn't follow a drop in vacuum.
I've removed the cruise/reservoir/brake pedal vacuum line from the back of the check valve off the back of the plenum and plugged that part of the check valve, thus removing that portion of the vacuum system - same results. So one of those things isn't leaking.
I had been thinking EGR all along - I can actually *fix* it, by unplugging the MAF. Car actually runs a good bit *better* without the MAF plugged in. I'm assuming this is because unplugging the MAF cuts off some other parts of the emissions system?
Today I got to see for sure as I have a vacuum gauge. Its not the car actuating EGR - and it being a weak spring opening the valve too much as I monitored EGR vacuum when it occurred and no vacuum was present. EGR vacuum works, as I rev the motor and get about 1 inch of vacuum on the downside. So the EGR solenoid and switch are working.
Leaving EGR unplugged and the vac port plugged up results in a 32, that goes away when I clear the codes and plug it back in. So I believe the EGR itself isn't bad. I suppose it could be stuck open, but actuating the valve changes the motors idle, so I don't think so. I could be wrong here.
Only codes I've ever seen are 15 (I think I caused that by shorting the ECT with the TB heater bypass (metal tie-cables aren't good for electrical cables!). 33 and 34. I know 34 is from having the MAF unplugged. I've not seen the 33 in a while either. So essentially no codes.
Any thoughts?
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