navy_vette

1991 with ZZ4, Miniram, Stage 3 TPIS tune, Miniram, Edelbrock heads.

This is freaking me out. I started getting misses and bumbling under load. When accelerating, it starts. It happens across almost the entire RPM range. Under light accelerating, it starts to bump for a second and then suddenly straightens out. I thought it was fuel starvation, so I put in a new fuel filter. No good. So, I changed spark plugs, went thru all of the vacuum lines, vacuum tested all of the components. Good to go. So, there are zero vacuum leaks. So, I checked timing. Pulled the EST and started her up. 6 degrees BTDC, right on the money. Pulled the battery, connected EST, etc... Started it up, solid on the mark. Hit the gas.... no good. The timing shifted maybe 10-12 degrees. Never went past that. So, the timing is bad. All distro connections are good.

I'm looking for things to check. Ideas. Sensors? Knock sensor? Bad advance components in the distro? Anyone see anything like this before?

Thanks for the help.

jv9999

The fact that the advance is limited is not a reason for your missing under load. It should not miss even with no advance. A new fuel filter doesn't mean you have good pressure. Put a pressure gauge on it and watch it under load.

If it's good, then it's time to do some data logging before you start replacing stuff.

Cliff Harris

The advance is normally controlled by the ECM, but it is possible that the ECM is not controlling the advance for some reason. In that case the ignition module takes over and has a limited advance curve built into it.

You need to data log to find out what is really going on.

navy_vette

I don't have a logger, but my mechanic does. I already talked to him about doing by today, but I will have to wait for this rain to stop. I'm on San Diego and it is pretty insane right now. I'll also try to reseat the chip before I drive over there. It sounds reasonable that the controller has taken over with the ECM falling to do it's job.
I've tested fuel pressure. It's good. I also did basic maintenance and replaced plugs (color was good, but they were old and as bit burnt) and cap and rotor (very VERY bad). It's stronger, but the problem is still there.

joe paco

I have not been under the hood lately, but last time I tried to check total advance as you did, I got the same reaction, 10-12 advance. The timing was never an issue so I didn't bother explaining it. But, with the ecm I don't think it is possible to check total advance like you can with a dist. Maybe that is not correct. If you have done it before, then I am in error about it. I did have the est connected when I tried it.
joe

Cliff Harris

You can check your total advance by adding a timing tape to your harmonic balancer:

htrdbmr

The ECM controls advance as a function of rpm, temp, and map. You may not see much advance in a "static" condition changing only rpm.

pcolt94

Check out the MAP sensor. It affects engine operation with timing and fuel. If it is not working properly, it can throw the timing all over the place.

The sensor is not easy to test for you need a external hand vacuum pump. You can leave it on the car for power and check the wiper for 0 to 5 volts output. But sometimes it's easier to just replace if you're not really set up to do this and know what you are looking for.

Just a suggestion but the MAP gets overlooked. They don’t go bad much but when they do symptoms can be confusing

ghoastrider1

throttle P sensor?

383vett

Cliff has a good point. You mentioned the timing "shifted" 10-12 degrees when the wire is connected. That is normal at idle. Does the advance change as you rev the motor. Also, you mentioned that you changed the plugs. Often the wires are the culpret.

navy_vette

I'll have to look at putting that tape on the balancer. Looks very useful. Then again, getting it on isn't easy, I'm sure.

So, I did basic maintenance, just to be sure, and took some steps to make sure fuel and spark are good.
Replaced:
Spark plugs
cap and rotor (very bad)
fuel injectors
all Vacuum lines
Tested all vacuum modules (brake booster and such) all good
Tested spark (all good)

I also took the car to the mech and hooked it up the the computer. All sensors are reporting and the ECM says it's advancing timing.

I have been getting a few posts about the ignition module being bad, but any time I've dealt with on of them they are either good, or the car doesn't work.

I'm also looking at the TPIS chip. Today, I'm going to pull it and put the stock chip back in. I can't really see how it would go bad, just being an EPROM, but it's possible. I already re-seated it, so...

Thanks for all of the replies.

navy_vette

DOUBLE POST

I know, it's annoying... but... I did the test I should have done from the beginning. I threw a multimeter against the Throttle Position Sensor. Open. Completely open. So, the computer thinks that the throttle is ALWAYS at idle. So, when I try to open it up, it runs VERY lean. But, when I baby it, it runs fine since it is running a bit rich to begin with.

I am currently trying to locally source a TPS. When I get one and try it out, I will re-post the results.


***edit***

Well, I was looking at the wrong setting. I was looking at ohms, not Kohms. It seems to be about 1.5k ohms lower than it should be, but it is a smooth change thru the whole sweep. Since the ECM calibrates itself right off the bat with the TPS, it shouldn't matter. It should be good. I'll do the dreaded compression test tomorrow morning. If i'm loosing compression in 1 or more pistons, then it looks like a rebuild... If not, then I'm stuck again.

pcolt94

I don’t see where you checked the fuel pressure, or did I miss it.

A check of the TPS with voltage is a better indicator. The wiper will go from (approximately) from 0.5 to 4.5 vdc as the throttle is opened (key on, no run). (sounds like your OK but voltage is what the ECM sees which is the important factor).

The MAP works the same except on vacuum pressure. See if you can back probe it on the wiper and with the engine running, open the throttle and see if the transition in voltage is smooth. (A hand vacuum pump works well for a test like this, like for brakes).

383vett

plug wires?

navy_vette

I'll test out the MAP. I have a vacuum test gun. I didn't even think about checking the resistance on the MAP while using the tester. Great idea! The MAP is holding vacuum, but I don't know if it is reading correctly.

I'll replace the plugs in the morning when I pick up the new TPS (I'm going to replace it anyways) and a new coil (just in case).

I am very hesitant to blame it on a sensor. If it was, I should be getting a code. It's still worth testing. I'm thinking it has to be an end-user module. I'm also going to replace the Ignition Module in the distro just in case.

I know it seems like I am throwing parts at it, because I am. Luckily, all of these parts are cheap, so I don't mind. Without codes, it's difficult to nail anything down.

GRKLGHTNG

Saw your thread and wanted to know if you had any luck?