1G Stock engine stumbles above 5500 RPM

[97egl]

With a car that is 99% stock I am getting a stumble above 5500ish rpm. I used to be getting black smoke out the back too which I knew was boost leaks. I fixed all the boost leaks I could find and have no bubbles coming from anywhere now. Takes about 90 seconds to leak to 0 from 15 psi. No more black smoke but still have the stumble. It is all stock except for bypassing the wastegate solenoid with a "metered" tee so I'm making about 14 lbs of boost instead of 10. I've changed the plugs, wires, and air filter. They were all pretty old but I still am getting the stumble. I don't beleive it's fuel cut because it will push thru the stumble but very poorly/slowly. Any time I've hit fuel cut in other cars it is like hitting a brick wall. I'm thinking it may be knock retard but since this car has a stock pcm I have no way to monitor this. Was going to back off timing or go to colder plugs and see if it gets any better but wanted to see if anyone had any suggestions first.

Thanks guys.

 

[SixBolt_16G]

How is your idle? Is it elevated enough that you might still have a leak somewhere that might not show bubbles, like an internal TB shaft seal leak?

 

[97egl]

How is your idle? Is it elevated enough that you might still have a leak somewhere that might not show bubbles, like an internal TB shaft seal leak?

Idle is about 650 once it is warm. The seals are both new on the throttle body shaft. Throttle body gaskets are brand new as well.

I'm pretty stumped as to what is going on with it. I have half a mind to weld an o2 bung into the exhaust and toss a wideband on the car. I'd assume under knock the wideband readings wouldn't really change and then I could try colder plugs or backing off the timing. If the wideband shows rich then the ignition is the issue and if it goes lean maybe the fuel pump is going out and can't supply enough fuel.

I hate cars without obd2 or tuning software. Not having any readings sucks

 

[steve]

Try lowering the boost by running a vacuum line directly between the j-pipe port and the wastegate to start then you'll know if it's boost related.

If the IPS/ECU/ISC are working correctly you should have an idle of 750 or so and it's shouldn't drop when you turn on the headlights of the HVAC blower.

Stock 450cc injectors may be reaching their limits once you push the boost up and the 14b is going to be pushing more hot air at 14psi than at 10.

What/which spark plugs wires did you get?

 

[chrysler kid]

On a 32 year old car, I would check fuel pressure readings, or atleast take a peek at the fuel hanger.

 

[1990TSIAWDTALON]

If it is a stock MAF car, make sure the intake tube isn't sucking closed after 30+ years under higher rpms.

 

[Mech Addict]

High rpm stumbles on older cars makes me think of ignition trouble. Bad coil. When you say it will push through the stumble, you mean it's fine at speeds above 5500?

 

[97egl]

Thanks for all the replies and ideas guys!

Try lowering the boost by running a vacuum line directly between the j-pipe port and the wastegate to start then you'll know if it's boost related.

If the IPS/ECU/ISC are working correctly you should have an idle of 750 or so and it's shouldn't drop when you turn on the headlights of the HVAC blower.

Stock 450cc injectors may be reaching their limits once you push the boost up and the 14b is going to be pushing more hot air at 14psi than at 10.

What/which spark plugs wires did you get?

If I run a vacuum line directly from the j-pipe nipple to the wastegate it will smoothly run up to redline but seems not to have much steam above 4500 or so rpms so thinking there's still an issue but just not as noticeable with less boost. But maybe I am just used to the higher boost so it seems weak. I guess I should try to "re-meter" the tee and bring the boost down lower but not way down to 5 psi

I have NGK Plugs (BRP6ES) and Napa Gold wires (rebranded Belden Max) I've always had very good experience with them in the past.

I know the ISC is not the greatest. This car had a better one but I pulled it to put on my daily so that car would have a better idle. This one likes to hunt until it warms up and then kinda surges between 600 and 700.

On a 32 year old car, I would check fuel pressure readings, or atleast take a peek at the fuel hanger.

Fuel pressure is good at idle. I don't have a gauge I can run inside and watch when it has the issue tho.

If it is a stock MAF car, make sure the intake tube isn't sucking closed after 30+ years under higher rpms.

Yeah I read about that and put hose clamps around the tube but that didn't seem to make any difference. I really thought that was going to be it so was quite disappointed when that didn't help.

High rpm stumbles on older cars makes me think of ignition trouble. Bad coil. When you say it will push through the stumble, you mean it's fine at speeds above 5500?

I can get it all the way to redline but it stumbles on the way there and is doesn't have a lot of power. I had thought ignition coil issue so i dropped the plug gap down to .025 when i replaced the plugs to see if it would make a difference but had the same issue.

 

[DSMPT]

Besides what everyone mentioned, I would double check for the base ignition timing and fallen rocker arm.

 

[97egl]

Besides what everyone mentioned, I would double check for the base ignition timing and fallen rocker arm.

I have checked the base ignition timing and even backed it down some from there but it doesn't seem to make any difference. I will pull the valve cover and check the rockers this afternoon. Hadn't thought of that before but if only one valve were opening it would make sense that it would run decent down low and only be affected in the higher RPM

 

[1990TSIAWDTALON]

Is this an E85 motor, 10:1 or more compression. Just inquiring.

 

[chrysler kid]

Wait, so you dont have a boost gauge either?

Lets go back to stock and work back up with known variables instead of unknown.

I have had pumps with a failing o-ring or feed line that will work fine but hesitate in boost

Ive also seen bad grounds cause spark issues, can you verify the ground strap on the intake manifold is good and clean and that the negative terminal grounding to the firewall is good?

The next things to check are resistance at the coil and the ptu to make sure they are properly functional

 

[97egl]

Is this an E85 motor, 10:1 or more compression. Just inquiring.

No this is a 100% stock 6 bolt 1g engine except for the balance shafts are deleted and the wastegate solenoid is bypassed (I have a line running directly from the nipple on the air filter can to a tee that is between the wastegate and j-pipe) Everything else is stock.

Wait, so you dont have a boost gauge either?

Lets go back to stock and work back up with known variables instead of unknown.

I have had pumps with a failing o-ring or feed line that will work fine but hesitate in boost

Ive also seen bad grounds cause spark issues, can you verify the ground strap on the intake manifold is good and clean and that the negative terminal grounding to the firewall is good?

The next things to check are resistance at the coil and the ptu to make sure they are properly functional

I do have a boost gauge. The only thing not stock is that I have bypassed the wastegate solenoid by running directly from the nipple on the air filter can to a tee that is between the wastegate and j-pipe. I can change the amount of boost by putting smaller or larger vacuum line fittings on the line running from the air filter can.

I cleaned all the grounds when i got the car running about 2 years ago and they looked good but i will check them with a meter. I have already checked coil resistance as well as dropping the plug gap down to .025 from .032 which I believe should have made the issue at least better if a coil was failing. I had a bad coil when I was on a road trip in a different vehicle and made it back home by lower the plug gap which leads me to believe this should have at least helped.

I have not checked or swapped the PTU. I've not had issues with one before so didn't think of that but I can take one from another vehicle and try that! Also probably see what the service manual says for checking them. Thanks!

Car also does it no matter what the temp is. It will do it when the engine has hardly warmed up or when it's been running and fully up to temp.

 

[dwb]

Here are some of the things that have given me stumbling issues on my stock 1g in the past (I know you have already addressed some and others have mentioned some as well):
Spark plugs and wires
Boost leak (I've also had some pretty big leaks that didn't cause stumbling)
Pcv system
Low output from alternator
Low compression/blow-by
Too much boost
Knock sensor failure
CAS failure (not necessarily rpm or boost related)

You might also inspect your grounding points, fuel pressure, and TPS.

I assume no check engine light? How do your plugs look?

 

[97egl]

Here are some of the things that have given me stumbling issues on my stock 1g in the past (I know you have already addressed some and others have mentioned some as well):
Spark plugs and wires
Boost leak (I've also had some pretty big leaks that didn't cause stumbling)
Pcv system
Low output from alternator
Low compression/blow-by
Too much boost
Knock sensor failure
CAS failure (not necessarily rpm or boost related)

You might also inspect your grounding points, fuel pressure, and TPS.

I assume no check engine light? How do your plugs look?

Spark plugs and wires-New

Boost leak (I've also had some pretty big leaks that didn't cause stumbling) - Takes 90 seconds to go from 15psi to zero. I think that should be ok from what I have read. My other cars are better than that and take 3+ minutes but I'm pretty sure the valves are where I'm leaking on this.

Pcv system- Bypassed and vents to atmosphere
Low output from alternator- Idle is at 13.6 volts
Low compression/blow-by- I've got 155 150 140 150 psi for compression.
Too much boost- 14/15 psi doesn't seem like it causes others on stock systems and ive back it down to about 12 by metering the vacuum less that's on my "tee" setup
Knock sensor failure- put a new Mitsubishi brand one on a year ago
CAS failure (not necessarily rpm or boost related)


I haven't checked the CAS yet. I did start to get really long crank time and the check engine light finally came on yesterday but then it went out so hoping it comes back so I can get the code. I am really hoping the check engine light can lead me in the right direction.

The plugs are "whiteish" and maybe mildly ashy looking? But I'll be honest I have never been good at reading plugs. If it would be worth it I can pull them again and take a picture if someone on here would be able to tell me something from looking at them.

Thanks again for all the help guys!

 

[dwb]

the check engine light finally came on yesterday

There's your answer most likely. As long as you haven't disconnected power since the CEL it still has the code stored.

 

[97egl]

I didn't know the 1g's would retain and blink the codes even when the light is out. I haven't disconnected power so I'll try that. Hopefully I have my code testing bulb around somewhere that I can find it. Been a minute since I had to use it!

Thanks again and I will post what I find out.

 

[97egl]

Pulled code 21 out of it so going to check the coolant temp sensor ohms against the service manual and see what I get

 

[97egl]

Old temp sensor was bad when I tested it. Replaced it but still the same issue.

I finally had time to weld an 02 bung into the downpipe and put an AFR gauge on it. The readings start to steadily get leaner above 4500 and by the time it gets to 5500 its gone to 13.5:1 and then starts to stumble and the AFR goes all over the place.

So from that I am pretty sure I am running out of fuel. It's got stock injectors but I metered the boost down to 12 psi so it shouldn't be out of injector especially at 4500 rpm so my thought is the pump is worn out and not flowing enough fuel. Ive checked fuel pressure at idle and it's good so that tells me the regulator is good.

What are your thoughts guys?

 

[dwb]

Can you datalog it? Is the fuel pump rewired? Could still be a voltage issue. Have you tried with all the accessories off vs on?

 

[chrysler kid]

Old temp sensor was bad when I tested it. Replaced it but still the same issue.

I finally had time to weld an 02 bung into the downpipe and put an AFR gauge on it. The readings start to steadily get leaner above 4500 and by the time it gets to 5500 its gone to 13.5:1 and then starts to stumble and the AFR goes all over the place.

So from that I am pretty sure I am running out of fuel. It's got stock injectors but I metered the boost down to 12 psi so it shouldn't be out of injector especially at 4500 rpm so my thought is the pump is worn out and not flowing enough fuel. Ive checked fuel pressure at idle and it's good so that tells me the regulator is good.

What are your thoughts guys?

Its the fuel pump like I mentioned earlier

 

[steve]

Ive checked fuel pressure at idle and it's good so that tells me the regulator is good.

You don't know that the regulator is good until you have checked that it tracks manifold pressure. If it does from vacuum to what your boosting then you know its good. You can do this without the engine running using the fuel pump test connector to power.

I wouldn't be surprised if it was a tired fuel pump at this point.