87 TBI 350 thumpr cam

[Tbarr]

Hi all. Brand new to the forum. I recently purchased an 87 silverado shortbox with a 350. The previous owner had a thumpr cam installed. I have it in the garage to start going through it. I noticed it was running rich. I checked the MAP sensor, and it was unplugged. i plugged it in and now it wants to die. In the short drives i have taken, the brakes are spongey as well. So from some research, it seems that due to the cam, its not making enough vacuum. i was going to pick up a vacuum gauge and check and see what it is. I am not super familliar with the TBI. Has anyone put a thumpr cam in before? if so, how did you manage the vacuum to run everything correctly? TIA

 

[fast 99]

MAP is disconnected because of low vacuum. ECM thinks the engine is under a load. A disconnected map puts the ECM in a default. It will substitute to an internal algorithm. Vacuum will have to be raised in order for TBI system to operate properly, replace cam. Then additional troubleshooting can proceed. Very likely that won't be the end of it.

May be there is an aftermarket prom available that modifies the data similar to diesel reprogramers. Highly illegal but that's your decision.

 

[MarineOne]

I run thumper cams in my muscle cars I use dual diaphragm boosters from like a 78 ws6 trans am to boost the brake pressure back up. That said it probably only makes around 10lbs vacuum.
How does brake pedale feel with truck off in 3 pumps it should be nearly at top and solid with no fade. If it sags testing like that it needs repair or brakes bleed

Do an idle relearn

Plug map in
disconnect battery
Touch terminals together for 10 seconds
Reconnect battery
Drive truck this will reset its idle and fuel tables

Im not sure what else is vacuum on truck besides brakes but it's very love vacuum cam because of the 107° love separation angle

 

[MarineOne]

MAP is disconnected because of low vacuum. ECM thinks the engine is under a load. A disconnected map puts the ECM in a default. It will substitute to an internal algorithm. Vacuum will have to be raised in order for TBI system to operate properly, replace cam. Then additional troubleshooting can proceed. Very likely that won't be the end of it.

May be there is an aftermarket prom available that modifies the data similar to diesel reprogramers. Highly illegal but that's your decision.

Tuning a vehicle is not illegal anywhere in the United States. You can tune brand new diesel trucks too. It's only illegal to bypass factory emissions devices. And in many states / countries after a certain amount of years it nolonger needs to follow any standard

Federal standard for emission exemption is 25 years and Iowa has no inspection so he can do anything he wants and any engine modifications he wants

 

[fast 99]

Tuning a vehicle is not illegal anywhere in the United States. You can tune brand new diesel trucks too. It's only illegal to bypass factory emissions devices. And in many states / countries after a certain amount of years it nolonger needs to follow any standard

Would be surprised if that is true. Wouldn't want to put myself in a position to defend splitting that hair. Don't want to derail the OP's question so will leave it at that.

 

[MarineOne]

Would be surprised if that is true. Wouldn't want to put myself in a position to defend splitting that hair. Don't want to derail the OP's question so will leave it at that.

It's true rest assured

 

[bucket]

The TBI ecm will not ever like a Thumper cam, even with custom PROM burning. Talking to Brian Harris at TBI Chips is probably your best bet.

 

[Vbb199]

Those cams make alot of noise, but otherwise suck. Im sorry, but they do.

Pull it for something with better vacuum. creating a EPROM from someone remotely for a thumpr cam is gonna be real difficult to get dialed in just right.

It'd be better for in person tuning, but goos luck finding someone to do obd1 tuning anymore.

As far as the tuning, yes its legal to tune
Yes you can tune brand new diesels.
HPTuners wouldnt be providing PCM/BCM support for them if you couldnt, rest assured of that. Theyre all in bed with the EPA.

Yes in many states, after a certain yesr they dont even care. Take NC for example, after 03, they dont give a darn


You can try increasing timing with the distributor and raising idle speed manually for the cam (letting more air in) but honestly without a tune its gonna misbehave im sure.

 

[Hunter79764]

Quick and dirty disconnecting the MAP is probably the fastest way to a usable truck, but won't be "right". x2 on PROM tuning, a few folks still do it, but it has limitations for sure. And I hate to weigh in, but take a look at HPTuners etc, they are very clear it is for off-road and track only cars, or for diagnostics etc. on street cars. If you use it otherwise, that's on you, your state may not care but the federal gov't still does, if they ever find a way to enforce. Past that, the less anyone talks about it, the better...

 

[Hunter79764]

Oh, and Vacuum canister and/or dual diaphragm boosters are the common fix for your issues. Past that, if you are committed to a nasty cam, Hydroboost gets rid of the vacuum assist and uses PS pressure to boost your brakes. You can adapt it fairly easy, but you do have to know what you are doing or buy from a reputable vendor that knows what they are doing.

 

[Craig Nedrow]

A 107 LSA allows the intake and exhausts to be opened at the same time on the overlap. This is good and bad. Bad, less MPG, low vacuum. Good, allows the piston to help excavate the exhausts gasses and pull in the intake and can create more HP if tuned right, Sounds cool. Soooo if you want power, 107 LSA is better then 114 or 116. The latter will up MPG, good vacuum, generally less power.

 

[potent rodent]

all those thumper cams are good for are noise. get that out of there and put in a stock cam or if u want a nice little lope put in a rv cam thumper cam makes no power

 

[Trucksareforwork]

Just to pile on the cam topic because I have nothing to offer in the TBI department: The 1980 truck I have (Nemo) came with a mutha thumper cam in it. After fiddling with it a little bit I pulled it. Terrible when cold, no vacuum (like 5-7 inches iirc), and really not much power.

I would re cam and avoid chasing this thing forever. What’s the point of a truck that sounds like a hot rod but is gutless and crappy to drive to boot? I guess there was a market for that and comp found it.