Power surge at 2% to 90% throttle..

[capt_yoda]

My 1987 Chevy R20, 350ci, TBI has no power and surges after 2% throttle and doesn't gain power back till after 90% throttle. The TPS is new and checked with multimeter, no dead spots.. New Fuel Pump and tested at the back of the TBI@14psi while running, 12psi@load.. New Fuel Regulator, New Fuel Dual Tank switch valve, New Cap/Rotor/Wires/Coil/Module,... Tested spark, is light blue and strong... In all, she starts and idles great and when full throttled, but under normal driving, she runs like a 5-6 cylinder... No backfiring, from intake or exhuast, just won't run strong unless you floorboard the throttle.. All mechanics I know don't have a clue to the problem,...NEED HELP PLEASE..

 

[CSFJ]

My 1987 Chevy R20, 350ci, TBI has no power and surges after 2% throttle and doesn't gain power back till after 90% throttle. The TPS is new and checked with multimeter, no dead spots.. New Fuel Pump and tested at the back of the TBI@14psi while running, 12psi@load.. New Fuel Regulator, New Fuel Dual Tank switch valve, New Cap/Rotor/Wires/Coil/Module,... Tested spark, is light blue and strong... In all, she starts and idles great and when full throttled, but under normal driving, she runs like a 5-6 cylinder... No backfiring, from intake or exhuast, just won't run strong unless you floorboard the throttle.. All mechanics I know don't have a clue to the problem,...NEED HELP PLEASE..

I've got a couple of these tbi's now, and I'm no expert, but could this be a symptom of a bad coolant sensor? I have heard they can cause all kinds of driveability issues when bad.

 

[capt_yoda]

I've got a couple of these tbi's now, and I'm no expert, but could this be a symptom of a bad coolant sensor? I have heard they can cause all kinds of driveability issues when bad.

Yes, that's accurate, although I have already replaced it as well, along with the IAC, however none of those were related to this problem but other problems when I first got this truck..

 

[chengny]

No SES light or recent codes thrown? IDK, maybe check the MAP sensor connections (hose & harness).

 

[CSFJ]

Timing where it should be?

 

[capt_yoda]

Timing is dead on... and no SES codes, it has had 3 months to store codes since I last disconnected the battery... also had it connected to scanner 3 days ago and all sensors are communicating with ECM without issues..

 

[capt_yoda]

As for MAP sensor, the scanner did show it was a little low, but not enough to cause this.. and not low enough to throw a code either...

 

[capt_yoda]

Also, all Vacuum lines have been replaced and routed correctly... same for fuel lines... and fuel filter...

 

[capt_yoda]

The problem has been there since I bought the truck 2 yrs ago... but it had many other problems before I got it dialed down to this last problem... it wasn't this bad till a week ago, it was intermittent and would come and go once in awhile, but now it is constantly doing it... making it difficult to get to work everyday, about 8 miles round trip daily...

 

[1987 GMC Jimmy]

I know I'm like the EGR valve schoolmarm around here, but I didn't hear mention of it. It sounds like you've got good spark, good fuel, and good air. I imagine you've checked for vacuum leaks. Anyways, back to the EGR. I had that go out on the Jimmy, and it ran worse and worse until it didn't run at all. The old style ones will have an exposed plunger on the underbelly of the valve. Push that down and make sure it travels good. If it does, then push it all the way down, cap your finger over the nipple, and let go of the plunger it should stay there. The newer valves work the same, except the plunger is enclosed in a clamshell with holes along the bottom. You'll have to remove the valve in this case and stick a narrow instrument in the holes to perform the same test as aforementioned. Also, if you take it off, be sure and clean the carbon off the pintle. So you haven't gotten any fault codes? The only thing that's happened to me that sounds similar is the MAP vacuum line melted, but my EGR woes also sound similar.

 

[capt_yoda]

I had too much new stuff to post it all together, but I had checked and replaced the EGR valve last year.. it is functioning correctly... melted vacuum line from MAP sensor? How did that happen, unless the line was against the EGR?

 

[1987 GMC Jimmy]

I had too much new stuff to post it all together, but I had checked and replaced the EGR valve last year.. it is functioning correctly... melted vacuum line from MAP sensor? How did that happen, unless the line was against the EGR?

Precisely. I had changed the EGR valve, and the MAP line was touching the valve. Well, damn. Are you getting an ECM self-test when you first turn the key on, and can you pull the codes? I had an ECM issue on the Caprice almost a year ago, no self-test and couldn't pull codes, so I pulled the ECM and one of the integrated circuits had burned. It's CCC not TBI, but it's pretty much the same thing. It would idle okay, and it would cruise okay, but everything in between was crap. And the problem wasn't a day and night thing. It got worse until I had to diagnose it and fix it.

 

[capt_yoda]

Never had that happen before, nor this issue I'm having now and I've had 5 other TBI equipped vehicles.. As stated earlier, there is no stored codes, self diagnostics check out with no problems... SES Lights up when turning on the key... The ECM and Prom are both new... Scanner check was good across the screen for all, accept the slightly low reading for the MAP sensor reading...

 

[CSFJ]

You're completely certain that the plug wires are on in the correct firing order?

 

[1987 GMC Jimmy]

Never had that happen before, nor this issue I'm having now and I've had 5 other TBI equipped vehicles.. As stated earlier, there is no stored codes, self diagnostics check out with no problems... SES Lights up when turning on the key... The ECM and Prom are both new... Scanner check was good across the screen for all, accept the slightly low reading for the MAP sensor reading...

Sorry, I guess I missed the ECM stuff. You're breaking my heart here! The only thing I can think of besides what's either been said by me, you, or other posters is something with the ESC module or maybe your catalytic converter is getting plugged. You can check the pipe's upstream and downstream temps relative to the cat, and a large temperature drop would normally indicate something's fishy.