Ok here is the deal. I had some bearing squeal on a 4000 mile water pump. So this weekend I replaced it.

At the same time I replaced the coolant gauge sender sensor. The one between cylinders 1&3 in the head. My new heads had bigger sensor holes than stock, so I originally put in the wrong gauge sender. I also replaced the secondary fan switch with a low temp unit.

The install went fine except now my car is running like crap. The car ran fine before the install. Now once it warms up the car stumbles really badly with any throttle input. It also stumbles a little bit while revving up in neutral, after driving. It is basically undrivable. It revs up fine in neutral when first started. Shortly after starting to drive the problems begin.

Also, the car has a hard start problem that has been getting worse over the last 200 miles. I really don't think its the starter, as the motor spins over fast and clean, it just doesn't start. It takes about 2-3 tries, and it took about 5 tries the last time.

So, what I've tried.

1. I am getting no codes.

2. The TPS checks out ok.

3. I unplugged the MAF, it made no difference.

4. I swapped out the ECM with a 'new' one, seemed to help, but not much, as weird as that sounds

5. Base timing is set okay

6. fuel pressure checks out

Other considerations:

350TPI
EGR disabled in the PROM
EGR is still installed and hooked up, just disabled in the prom
No AIR
24lb injectors
Injectors have less than 2,000 miles on them and check out ok
700R4 trans
Lingenfelter TPI manifold
AS&M runners
ZZ4 cam
Trick Flow 195 heads
headers, cat, catback

Any thoughts?

Thanks guys,

John

 

It should run worse when you disconnect the MAF, it would not run good then, it would run but not good.. Seems strange that the idle didnt change when unplugging the MAF..

Maybe your MAF is toast ? got any friend nearby that can borrow you the MAF to try it out ?

 

I should add I turned off the engine, unplugged the MAF then restarted the engine. So there may have been some difference, but nothing I could tell from the process I used. Should I try unplugging it while the engine is running?

 

Okey then it may have been some difference, you did get an SES light right when it was unplugged..

I think.. that you can pull it when engine is running. Cant remember how I did, youd better wait for atleast 1 more answer on this before you pull the MAF while running. but as said, I think its ok.. but wait and see what the other guys say to..

 

Yeah I may give that a try. It won't hurt anything to pull the connector while the engine is running. And yes, I do get the SES light when the MAF is unplugged.

 

Hey guys the car isn't with me now. I hopefully will have some time to work on it this weekend.

Does anyone have any other ideas of things I should check out?

Also, is there any scientific way to test the MAF?

Thanks

 

Unplug when engine is running ?

No you could prolly scan it with datamaster (or anything else) and see what /gramsec it flows and if it looks like an good value, also for dips if it runs 10grams/sec and bumps up to 90 and you havent done anything then its no good...

You can check to see if there are any debris inside the maf tube, and check for other things that are loose.. be carefull.

 

I don't think trusting me ear to detect a change is too scientific.

Unfortunately I don't currently have a good way to take data out of the ALDL port. Seems like I should be able to test voltage output of MAF, by jumping the connector. Does anybody know what a reasonable output voltage is at idle or any other rpm?

Or it seems like I should be able to test unpowered resistance in the MAF, the problem is I don't know what values are 'good' for the MAF.

BTW the MAF is visibly fine, the wires are connected.

There is a lot of documented cases where unplugging a MAF seems to improve a stumbling issue. This is not the case for me. For this reason, I don't think my MAF is bad.

Aside from the MAF, does anybody have any other ideas? Honestly I was suspecting the timing advance.

 

Ok I ran a few tests tonight.

I tried unplugging the MAF with the engine running. I did not notice any change in the engine running.

I also checked out the EST unit. I unplugged the EST wire, started the engine, and noticed the same hesitation while revving. So I'm guessing the EST is okay, sound fair?

I feel like the issue is timing related. Maybe something else in the distributor?
BTW
the cap, rotor, coil, wires, plugs have less than 1000 miles on them.

 

Have you checked your base timing ?
and have you checked if the timing is steady when having the EST connected, if you idling it should be pretty stable the timing..

 

As I said in my first post, the base timing is set correctly. Its basically impossible to test timing with a timing light if the EST wire is plugged in. Because its well advanced of the timing scale. Anyway I'm not worried about idle, the car idles fine.

 

I would seriously double check the CTS for the ECM.

That sounds like it is not connected properly and the ECM is always dumping fuel into what it thinks is a "cold" motor.

 

Yeah but you see if the harmonic balancer is steady or if it flexes. the timing should be steady when the rpm is steady and the load is steady, it should not shift soo much thats just to see that is pretty much locked on a timing example 20 deg on idle..

 

Yeah I was thinking that originally too. However the ecm appears to be reporting temp correctly, when I connected my friends laptop.

Let me add some details to what the car is doing. I will start driving and everything is okay, then shortly down the road, the the car will start bucking hard and hesitating bad. This only occurs while giving it any amount of gas. You can get off the gas and everything seems fine. Get back on the gas, same problems. Its bad even with very little gas pedal. After the problems start, the engine will even hesitate while revving in neutral. The bucking is fast, maybe 3-5 times a second. The fast rate led me towards suspecting the timing.

Thanks for the ideas

 

Very interesting.

I a little puzzled. I suspected the CTS because that is right next to the water pump.

any other sensors reporting unusual readings? If you have a laptop, might as well check all the value. (knock counts, MAT reading, etc)

What else did you mess with?
I think we might have to chalk it up to a coincidence that the water pump was installed at the same time.

The bucking sounds like it is spark related, figuring out what is changed is the hard part.

 

Yeah, my first thought was what did I unplug or mess with?
MAF, CTS area, ect.. But I to now feel like the water pump install is pure coincidence.

I must admit the hard starting problem started before the water pump install. However the car still ran fine.

The laptop is actually not mine. I only had access to it for a short time. But I made sure to confirm CTS, TPS, No knock, unfortunately I did not check total timing while revving.

The behavior to me feels like too far retarded timing. So I suspected:
EST, Distributor, dist pick up coil. I unplugged the EST and the car reacted no differently while revving the motor in my garage, same hesitation. Of course when unplugged the EST tan wire, the idle did get slower, so it looks like it is working for at least idle advance.

Maybe I'll pull the EST this weekend and have it tested.

 

Update.

The distributor pick up coil tested okay according to what I have found in a search.

830 ohms between the two leads
infinite resistance between each lead and ground.

I installed a new ignition module in the distributor. Its really dumb, now the problem is worse. Now if I try to hold an rpm in neutral, the rpm will quickly drop. I didn't think the new module would make things worse thats for sure.

Also the inside of the cap was corroded, which I find strange since it only has a few thousand miles on it. I replaced it.

Any ideas?

Bad ignition module out of the box?

bad coil?

bad pick up coil?

something else?

Thanks

John

 

The original ignition module tested good at autozone. So I will put that one back on. My suspects are:
ignition pick up coil
main ignition coil

Any ideas why I have excess corrosion on the distributor cap after only ~4000miles?

 

What brand on the distributor cap ?
Some brands tend to have fast buildup of corrusion..

 

Its a GM part, probably a Delco.

 

Any pics of the buildup / corrusion ?

 

pic

 

I think that seems little to much how many years are it ?
Is it moist where you use the car ?
Do the rotor look the same ?

 

Agreed, thats why I mentioned it. This cap is about 5 years old. The factory original with 58,000 miles and 16 years old looked fine, it was replaced for the sake of completeness several years ago. The car lives in the same conditions as it has for the last 21 years. The cap is corroded because either:

1. its made with poorly chosen materials - unlikely
2. the ignition systems is not working properly - likely

The question is what and why isn't it working properly.

The rotor looks new.

 

How your blaster coil ? stock one or aftermarket?

Sparkplug wires ? new ? old ? brand ? Are they within ohm range ?

How do the sparkplugs look like, right colour of them ?

What happen if you change the dist cap ? does it get any better ?

 

I have no idea of the condition of the coil. Any way to test? It has about 5000 miles on it. It is a Delco replacement coil.

The wires also have about 5000 miles on them. They are Taylor wires. They are not burnt, and are flexible. I know what it feels like to have a burnt wire, this is different. To be honest I haven't checked the resistance on them. Due to the sudden nature of my problems, I would tend to think this isn't a wire breaking down over time, type of a problem.

The plugs are a little sooty, probably due to my untuned engine. They have been like this for years and the car ran fine. If I wipe off the soot they have a nice tan color.

Changing the cap makes no difference.

Thanks

 

I am interested in what you find because I have almost the same situation. I have some rust in the pick up area and always wondered if this could cause issues. Keep us informed of what you find out.

 

sounds like the distributor isn`t set exactly in the spot it needs to be in.....look for rust on that base....ecm could be taking a dump and if you pull the brain out and start the car, hold the brain in one hand and splap it with the other.....if the car hesitates and wants to die while it`s idling, it`s the brain and prom........had this problem and that fixed it.....good luck

 

gekko, thanks for the ideas

I did try a new ECM, the condition is no better. Don't know about the PROM though. Is there any way to test the PROM?

I haven't moved the distributor is 3 years and its been running fine during this time. I also checked base timing and it is ok. There is no rust on the distributor.

Thanks for the ideas and keep them coming.

 

There is no way of testing the prom exepct scanning it and read the values, you prolly gotta get an error code 51 if the PROM is bad, and then it would run really bad and even die at some points..

is it 6 deg you have on the timing ?

 

prom can take a dump at any time.....it costs more than the brain but makes the car come to life

 

Base timing is set at 8 degrees. Which is correct for the tune that in on the motor.

 

What happens if you set it at 6 and little higher ? does it get worse or better!?
You have checked so the timing is steady, not flexing around alot on idle...?